P3 Flashcards
When does induction not happen?
Induction does not happen if the conductor moves in the same direction as the magnetic field.
What happens if an electrical conductor cuts through a magnetic field?
If an electrical conductor such as a wire cuts through a magnetic field, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, an electric current will flow in the circuit.
How can the induced potential difference be increased?
moving the magnet or wire faster
using a stronger magnet
increasing the number of turns, or loops, on the coil
increasing the area of the coil
What produces a magnetic field?
When an electric current flows through a wire, it produces a magnetic field around the wire. This magnetic field is only present while the current is flowing.
How do you increase The size of the force on a wire carrying a current in a magnetic field?
increasing the size of the current
increasing the strength of the magnetic field
What needs to be present for induction to happen?
For induction to happen, the conductor must cut through the magnetic field. This can be achieved in two ways:
a conductor can be moved in a magnetic field a magnet can be moved in a coil of wire
How do you increase the speed of the motor?
The speed of a motor can be increased by either increasing the size of the current or by increasing the strength of the magnetic field.
How do you calculate the potential difference across the coils in a transformer?
Vp/Vs = Np/Ns
Vp is the potential difference across the primary coil in volts, V
Vs is the potential difference across the secondary coil in volts, V
np is the number of turns in the primary coil
ns is the number of turns in the secondary coil
What is the differences in power usage when no load is applied in switch mode transformers and iron core transformers?
Switch mode transformers use very little power, whilst iron core transformers use the Same as if a load was being applied because a current continues to flow through the primary coil
How do loudspeakers work?
The electrical current from the amplifier is continually changing direction which, in turn, causes the magnetic field around the electromagnet to continually change.
The changing attraction and repulsion between the permanent magnet’s magnetic field and the electromagnet’s magnetic field make the electromagnet move back and forth.
In turn, the speaker cone vibrates back and forth, which generates sound waves. The frequency at which the current changes direction is the frequency of the sound that the speaker produces
Why must the direction of the current in a simple electric motor be reversed every half-turn?
The direction of the current must be reversed every half turn, otherwise the coil comes to a halt again. This is achieved using a conducting ring split in two, called a split ring or ‘commutator’.
Why are transformers not 100% efficient in real life?
Some energy will be lost to the surroundings as heat from the iron core and the coils.
What is the difference of size in switch mode transformers, and iron core transformers?
Switch mode transformers are Relatively small and light, whilst Iron core transformers are Relatively large and heavy due to the iron core)
Why are switch-mode transformers needed?
Some devices need a smaller potential difference than the 230 V from the mains electricity. Therefore, they need a step-down transformer to reduce the potential difference, built into the plug or power supply.
What are the advantages of using switch-mode transformers?
Switch mode transformers use very little power when the plug is left switched on but no load is applied (such as when the device’s batteries are not charging). This is another advantage for using switch mode transformers in applications such as mobile phone chargers.
Assuming that the transformer is 100% efficient how can you work out the power output of each coil?
he power output from the secondary coil will be the same as the power input to the primary coil. This can be shown by the equation:
Vp × Ip = Vs × Is
Vp is the potential difference across the primary coil in volts, V
Ip is the current in the primary coil in amperes (amps), A
Vs is the potential difference across the secondary coil in volts, V
Is is the current in the secondary coil amperes (amps), A
How do you change the direction in which the electric motor turns?
The direction in which an electric motor turns can be reversed by reversing the direction of the current, or by reversing the direction of the magnetic field.
How do switch-mode transformers work?
by using complex electronic circuits. These rapidly switch the current on and off, allowing the alternating current to be changed to a higher frequency. This is often between 50 Hz and 200 Hz. At these frequencies, a much smaller and lighter transformer than normal is able to reduce the potential difference. As a result, these transformers are suited for use in power supplies such as mobile phone chargers.
How do you create a simple electric motor?
A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves. One side moves up and the other side moves down (based on Fleming’s left hand rule).
How do electric bells work?
When the current flows through the circuit, the electromagnet makes a magnetic field.
The electromagnet attracts the springy metal arm.
The arm hits the gong, which makes a sound.
The circuit is broken now the arm is out of position.
The electromagnet is turned off and the springy metal arm moves back.
The circuit is complete again.
How do transformers work?
An alternating current passes through the primary coil.
The alternating current produces a magnetic field that continuously changes direction. The soft iron core increases the strength of the magnetic field.
The secondary coil cuts through the changing magnetic field, inducing an alternating potential difference across the ends of the coil.
An alternating current flows if a circuit is connected to the secondary coil
How do circuit breakers work?
A spring-loaded push switch is held in the closed position by a spring-loaded soft iron bolt.
An electromagnet is arranged so that it can pull the bolt away from the switch.
If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the push switch into the open position.
How do you work out the direction of the force?
using Fleming’s left hand rule.
To do this, spread out your left thumb, forefinger (index finger) and second finger so they are all at 90° to one another:
point your forefinger (index finger) in the direction of the magnetic field (north to south)
point your second finger in the direction of the electric current (positive to negative)
Your thumb will point in the direction of movement. Note that the direction of the force is reversed if either the direction of the current is reversed, or if the direction of the magnetic field is reversed.
What is the differences of frequency in switch mode transformers, and iron core transformers?
Switch mode transformers Operate at a high frequency, often between 50 Hz and 200 Hz, whilst Iron core transformers Operate at 50 Hz (UK mains frequency)
What does the size of the force depend on?
This size of the force is greatest when the wire is perpendicular to the magnetic field of the permanent magnet. In other words, it cuts through the magnetic field at 90°. If the wire is parallel to the magnetic field, it will not experience any force.
What is the motor effect?
The effect that occurs when a current carrying wire in the presence of a magnetic field experiences a force.
How can you tell the difference between a step up and a step down transformer?
step-up transformers have more turns on their secondary coil
step-down transformers have more turns on their primary coil
What are electromagnets?
Wire is wrapped around a soft iron core, and an electric current passed through it. The electromagnet behaves as if it were a bar magnet, except that it can be switched on and off.
What is the structure of a transformer?
A transformer consists of a soft iron core with two coils of insulated wire wrapped separately around it. Each coil has a different numbers of turns, or loops.
The primary coil is connected to an a.c. supply. It acts like an electromagnet. The secondary coil is where an alternating potential difference is induced. It is important to note that there is no electrical connection between the primary and the secondary coils.
What is inside loudspeakers?
Loudspeakers transform electrical signals into sound. Inside a loudspeaker there is a permanent magnet. An electromagnet attached to the speaker cone is inside the magnet field of the permanent magnet.
What factors affect centripetal force?
he centripetal force needed to keep an object moving in a circle increases if:
the mass of the object increases
the speed of the object increases
the radius of the circle in which it is travelling decreases
What is the time period for a pendulum?
The time period for a pendulum, T, is the time taken for a pendulum to swing from one side to the other, and then back again to its original position.
How do you calculate the time period for a pendulum?
T = 1/f
where:
T = time period in seconds, s
f = frequency in Hertz, Hz
How do levers work?
Levers rely on the principle of moments to act as ‘force multipliers’ - they reduce the effort needed to move the load by increasing the distance over which it is acting. This means a relatively small effort force has a much greater effect.
What is a moment?
A moment is the turning effect of a force around a fixed point called a pivot.
What are some examples of pendulums?
swings at playgrounds
some fairground rides - eg pirate ship rides
the inside mechanisms of some clocks - eg grandfather clocks
What must an object be if it is not turning around a pivot?
Where an object is not turning around a pivot, the total clockwise moment must be exactly balanced by the total anti-clockwise moment. We say that the opposing moments are balanced:
sum of the clockwise moments = sum of the anti-clockwise moments
What is the centripetal force?
hese objects continuously change direction as they move in a circle. This needs a resultant force to act on the object. This force is the centripetal force. The centripetal force pulls an object toward the centre of the circle.
Why does increasing the mass of an object increase the centripetal force needed?
Remember: force = mass × acceleration
To maintain a particular circular motion, there will be a particular acceleration. An object with more mass must have more centripetal force acting upon it.
How do you find the centre of mass for symmetrical objects?
The centre of mass for a symmetrical object can be found easily. The axes of symmetry are marked on the object. The centre of mass is where the axes of symmetry cross.
What is the time period of a pendulum dependent on?
The time period of one swing of a pendulum is dependent only upon the length of the pendulum and not upon the mass of the bob, or how high it swings. Longer pendulums have greater time periods than shorter pendulums.
Why are liquids hard to compress?
Particles in liquids are close together, making liquids virtually incompressible.
How do you calculate pressure?
P= f/a where: P = pressure in pascals, Pa F = force in newtons, N A = cross-sectional area in metres squared, m2
What is the line of action?
Weight pulls from an object’s centre of mass in a vertical direction toward the Earth. This is known as the line of action of the object’s weight.
How are cranes designed to prevent them toppling over?
Construction cranes lift heavy building materials using a horizontal arm called a jib. To prevent the crane toppling over, concrete blocks are suspended at the other end of the jib. They act as a counter-weight to create a moment that opposes the moment due to the load.
What is the centripetal force provided by when a vehicle turns a corner?
For a vehicle turning a corner, the centripetal force is provided by friction between the tyres and the tarmac.
Why is the pressure in liquids equal in all directions?
As the particles move around, they collide with other particles and with the walls of the container. The pressure in a liquid is transmitted equally in all directions, so a force exerted at one point on a liquid will be transmitted to other points in the liquid.
What is the stability of an object affected by?
The stability of an object is affected by two factors:
the width of the base of the object
the height of its centre of mass
Objects with a wide base, and a low centre of mass, are more stable than those with a narrow based and a high centre of mass.
Why does a small radius lead to an increase in the amount of centripetal force needed?
A circle with a smaller radius has a smaller circumference. Therefore, an object travelling in a circle with a smaller radius has less distance to travel per orbit. It will complete more of the orbit per second, changing direction by a greater angle each second. A greater centripetal force is needed to achieve this bigger acceleration toward the centre.
What are levers?
A lever is a simple machine that makes work easier to do. Levers reduce the force needed to perform these tasks. When someone uses a lever, they exert a force (the effort) around a pivot to move an object (the load).
How do you calculate the force or distance that must be exerted on one side of a pivot in order to balance out the moments?
Step 1: Work out the moment for which you have been given all of the information
Step 2: Change the subject of the equation to calculate the force or distance.
What else also uses hydraulic systems?
lifting equipment - eg hydraulic jacks and wheelchair lifts
lifting and excavating arms on machinery such as diggers
hydraulic presses - which are used during the forging of metal parts
wing flaps and some rudders on aircraft and boats
What is the frequency of a pendulum?
The number of complete swings (from one side to the other and back again) made by a pendulum per second is its frequency, f.
What do hydraulics systems do?
Hydraulic systems therefore allow smaller forces to be multiplied into bigger forces.
Why does increasing the speed of an object increase the centripetal force needed?
An object travelling faster covers more distance per second. It will change direction by a bigger angle each second compared to slower object. A greater centripetal force is needed to achieve this bigger acceleration toward the centre.
How do hydraulics systems work?
An effort force exerted on the plunger for syringe A puts greater pressure on the water in tube A. As water is virtually incompressible, the pressure is transmitted through the water into syringe B. The water pushes against the plunger in syringe B with equal pressure, exerting a load force on it.
However, tube B has a plunger with a bigger cross-sectional area than tube A. This means that the load force exerted is larger than the effort force exerted. This is known as a force multiplier
How do you find the centre of mass for irregular objects?
Drill a small hole in the object and hang it up so that it is free to swing without obstruction.
Hang a plumb line (a piece of string with a weight hanging from it) from the same suspension point. This lets you mark the vertical line directly below the suspension point.
Drill another hole at a different location within the object and then suspend the object with that hole.
Again hang a plumb line to determine the vertical and mark it on.
The point at which the two marked lines cross is the centre of mass.
Why do objects topple over?
As an object is tilted, the line of action will continue to pull down in a vertical direction. If the line of action moves outside the base of the object, there will be a resultant moment and the object will topple over.
What is stability?
Stability is a measure of how likely it is for an object to topple over when pushed or moved.
What is the centripetal force for objects in obit?
For objects in orbit, for example the Earth orbiting the Sun, the centripetal force is provided by gravity.
What is the centripetal force provided by?
Centripetal force does not exist in its own right, but is provided by the action of other forces. For example, imagine whirling a conker on a piece of string around in a circle. The centripetal force is the result of tension within the string.
Why are objects moving in a circle always accelerating?
An object moving in a circle is constantly changing direction. This means that, even if its speed stays the same, its velocity is constantly changing.If the object’s velocity is changing, it must be accelerating.
What is a simple pendulum?
A plumb line is an example of a simple pendulum. This is a simple machine consisting of a weight (called a bob) suspended from a suspension point by a thin piece of material such as string or a chain. The bob should be free to swing.
How are hydraulics used in the braking systems of cars?
The force from the driver’s foot (the effort force) exerts pressure on the brake fluid in a small piston. The pressure is transmitted throughout the brake fluid in all directions.
Next to each brake disc, there is a much larger piston with a greater cross-sectional area. The transmitted pressure acts on this larger area to produce a larger load force on the brake pads. The pads then rub against the brake discs and cause the car to slow down.
How do you calculate a moment?
A moment can be calculated using this equation:
M = F × d
where:
M = the moment of the force in newton-metres, Nm
F = the force in newtons, N
d = the perpendicular distance from the line of action of the force to the pivot in metres, m
What structure in the eye is the photosensitive surface similar to?
Retina - which detects light and converts it into electrical impulses which are sent to the brain.
What are the first three steps to drawing a ray diagram for a converging lens?
A principal axis is drawn through the centre of the lens at 90° to it.
The object is represented as an upright arrow (labelled ‘Object’) with its base on the principal axis.
The ray diagrams show the rays of light from one point on the object – they do not show all of the rays of light from every part of the object. These rays are drawn from the tip of the object.
What happens when light passes into a less dense medium?
When light passes into a less dense medium, it speeds up. The rays bend away from the normal. This means that the angle of refraction is greater than the angle of incidence.
What is the pupil?
Hole in the middle of the iris.
What happens when the eye focuses on a distant object.
The ciliary muslces relax, the suspensory ligaments stretch so the muscle tension on the lens is high. This causes the lens to become thin and less curved.
What does it mean if the magnification is below 1?
A magnification below 1 indicates that the image is smaller than the object.
What happens when an ultrasound reaches a crack or boundary and why is this useful?
When ultrasound waves reach a boundary between two media (substances) with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through. A detector placed near the source of the ultrasound waves is able to detect the reflected waves. It can measure the time between an ultrasound wave leaving the source and it reaching the detector. The further away the boundary, the longer the time taken.
What does the aperture in the camera do?
Allows the user to adjust the amount of light entering the camera in different light conditions.
What happens to the size of the image if the object gets closer?
is inverted and real - unless the object is at F or closer
What is the critical angle?
The critical angle is the angle above which total internal reflection occurs
What are cameras?
Cameras are devices that focus light onto a photosensitive surface using a converging lens. They have some similarities to the eye.
What is a CCD and what does it do?
A CCD is a charge-coupled device. Modern X-ray machines use CCDs instead of photographic film. The images are formed electronically, allowing them to be recorded and stored more easily than the images from photographic film.
How are CT scans used?
These are processed by a computer to build a three-dimensional (3D) image. This image can be manipulated in order to see the structures within the body at different layers and from different points of view. This lets a doctor gain a much greater insight into what is wrong with a patient.
How is accommodation achieved?
Accommodation is achieved by the contraction or relaxation of the ciliary muscles, which slacken or stretch the suspensory ligaments.
How can the distance traveled by an ultrasound be calculated?
s = v × t
where:
s = distance in metres, m
v = speed in metres per second, m/s
t = time in seconds, s
If you are finding how far away the boundary is, you need to divide it by 2.Why are ultrasounds useful for medical imaging?
The human body is composed of different tissues such as muscle and skin. Ultrasound directed at the body will be partly reflected at the boundary between these different tissues.
How do you work out the magnification of a magnifying glass?
magnification = image height/ object height
Note that magnification is a number without a unit.
How can ultrasounds be used in industry?
An ultrasound machine can be used to detect cracks or flaws in materials such as metal. This is used in industry for quality control procedures to check manufactured objects, such as railway tracks and oil pipelines, for damage or defects.
How is the power of the lens calculated?
Lens power can be calculated using the following equation
P = 1/f
where: P = lens power in dioptres, D
f = focal length in metres, m
What happens when light passes into a more dense medium?
When light passes into a denser medium, it slows down. The rays bend towards the normal (the normal is at 90° to the boundary between the two media). This means that the angle of refraction is smaller than the angle of incidence.
What is special about the power of the lens for a diverging lens compared to a converging lens?
he power of a diverging lens (used to correct short-sight) is negative, while the power of a converging lens (used to correct long-sight) is positive.
What is total internal reflection?
As the angle of incidence increases, so does the angle of refraction. Beyond a certain angle, called the critical angle, all the waves reflect back into the glass and no refraction occurs. This is known as total internal reflection.
What is the object and image distance measured in?
The object distance is measured in focal lengths : F, 2F (twice the focal length), and so on. The image distance can also be measured in focal lengths.
What does it mean if someone is long-sighted?
Someone with long sight can see distant objects clearly, but their point is further away than 25 cm. This means they cannot focus properly on near objects.
What does it mean if someone is short-sighted?
Someone with short sight can see near objects clearly, but their far point is closer than infinity. This means they cannot focus properly on distant objects.
How are ultrasounds used for pre-natal scanning?
These are widely used in pre-natal scanning to check that a foetus is developing normally and to take measurements of its growth. Computers can combine many ultrasound reflection readings to produce a detailed image from them.
What is the refractive index?
The degree to which a material slows the speed of light (compared to its speed in a vacuum) is its refractive index.
What is refraction?
Refraction is the change of direction of light as it passes from one medium (substance) to another. The two media must have different densities, such as air and glass.
What is the cornea?
Tough, transparent covering over the front part of the eye. Convex in shape.
How can ultrasounds be used to cure kidney stones?
Kidney stones are solid crystals formed from substances found in urine. They can sometimes build up into large stones inside the kidney. These can then pass into the ureter and cause a blockage, accompanied by severe pain. High frequency ultrasound waves focused at a kidney stone cause it to vibrate, breaking it into small enough pieces to pass out in the urine.
What is the iris?
Coloured part of the eye that contains muscles. These relax or contract to adjust the size of the pupil.
What can short-sightedness be caused by?
The eyeball being elongated - so that the distance between the lens and the retina is too great.
The lens being too thick and curved - so that light is focused in front of the retina.
What is one of the main differences between the camera and the eye?
One key difference between a camera and the eye is that a camera does not focus light onto the photosensitive surface by adjusting the shape of the lens. Instead, the focusing screws move the lens forwards or backwards in order to focus the image onto the photosensitive surface.
What does the amount of refraction depend on?
the angle the light hits the boundary between the substances
the difference in relative densities, between the two media (which determines how much the speed of light changes)
What happens to the orientation and the type of the image if the object is more the one focal point away?
is inverted and real - unless the object is at F or closer
What is a diverging lens and how is it represented?
A diverging lens is curved inwards on both sides. It is represented on diagrams by A diagrammatical interpretation of a diverging lens which is a vertical line with arrow heads at each end pointing towards the vertical.
What does it mean if the magnification is above 1?
A magnification above 1 shows that the image is larger than the object.
What does the photosensitive surface in the camera do?
Detects and records the light which is focused onto it. This can be on photographic film, but digital cameras use CCDs which convert light into electrical signals which can be stored.
What precautions are in place to protect hospital staff from the dangers of X-rays?
Hospital staff are also at risk from repeated exposure to low levels of X-rays. Shielded walls containing lead are built into all X-ray rooms to protect people outside the room. They have warning signs to show when the room is in use so that people do not enter.
How do you calculate the critical angle of a material?
refractive index = 1/sin c
where c is the critical angle in degrees, °
So: sin c = 1 ÷ refractive index
How are optical fibres used in endoscopes?
A doctor can insert a bundle of optical fibres into the body. Some carry light into the body, and some carry light reflected off internal body surfaces back out. This allows the doctor to see the inside of the body clearly – and help them diagnose diseases like cancer, or see what they are doing during keyhole surgery.
What are the suspensory ligaments?
Connect the ciliary muscles to the lens and hold the lens in place.
How are lasers used in eye surgery?
Lasers are used in eye surgery to repair damaged retinas and to correct vision defects. A laser can be used to precisely cut the cornea, altering its shape to correct a person’s vision.
What structure in the eye is the lens of a camera similar to?
Lens - which focuses light onto the retina.
What is cauterising?
In medicine, lasers are used to destroy damaged tissue or to stop bleeding, eg after surgery. This is known as cauterising
What precautions are in place to protect radiographers from the dangers of X-rays?
Only trained specialist staff - called radiographers - are allowed to use X-ray machines. They routinely leave the room, or stand behind a screen containing lead, whenever X-ray machines are in use. In situations where radiographers cannot stand behind a screen, they wear lead aprons which act as a protective layer of clothing.
What happens when the eye focuses on a near object?
The ciliary muscles contract, the suspensory ligaments slacken so the muscle tension on lens is low. This causes the lens to become fat and more curved.
What structure in the eye is the focusing screw similar to?
Ciliary muscles - which stretch or slacken the suspensory ligaments to adjust the shape of the lens
What are the ciliary muscles?
Muscles connected to the lens by suspensory ligaments.
What are the uses of lasers?
cutting through materials (such as metal)
burning (eg laser engraving) cauterising
What happens if the object is placed between the focal point and the lens?
If the object is placed between the principal focus (F) and the lens, a converging lens will produce a magnified, upright, virtual image. The image can only be seen by looking through the lens, and it appears on the same side of the lens as the object.
How can long-sightedness be corrected?
Long-sightedness is corrected by putting a converging lens in front of the eye, as shown in the diagrams below.
What does the optic nerve do?
The optic nerve carries impulses from the retina to the brain.
What does the lens in the camera do?
To focus light onto the photosensitive surface at the back of the camera. This can either be photographic film or a CCD (charge-coupled device).
What are the advantages of CT scans over traditional X-rays?
CT scans provide even higher quality images than normal X-ray photographs. The 3D images produced by CT scans allow analysis of different levels through the body without other structures obscuring the area of interest (as can happen with traditional X-ray imaging).
What structure in the eye is the aperture similar to?
Iris - which adjusts the amount of light entering the eye through the pupil.
What are the uses of optical fibres?
Optical fibres have become very important in high-speed communications, such as cable TV and high-speed broadband services. Information, in the form of pulses of light, is sent down bundles of optical fibres. Fibre optic cables are able to carry more signals than traditional copper cable telephone lines.
What happens to the position of the object as the object gets closer?
gets further away as the object gets closer - unless the object is at F or closer
What is a laser?
A laser produces an intense narrow beam of light. Laser light has a low divergence - which means that it spreads out very little.
How do you calculate the refractive index?
refractive index = sin i/sin r
Where:
i is the angle of incidence
r is the angle of refraction
How do you draw a ray diagram for a diverging lens?
Ray diagrams for diverging lenses follow similar conventions to those for converging lenses, but how the location of the image is determined is slightly different.
Three rays are drawn:
A ray passing parallel to the principal axis and being refracted away from the principal axis.
A ray directed at the principal focus behind the lens - but being refracted by the lens so that it runs parallel to the principal focus after leaving the lens.
A ray passing in a straight line through the centre of lens without being refracted.
What are the last three steps to drawing a ray diagram for a converging lens?
Not all rays from one point are shown. For simplicity, only three are shown:
a) A ray passing parallel to the principal axis and being refracted to a principal focus behind the lens.
b) A ray passing in a straight line through the centre of the lens without being refracted.
c) A ray passing through the principal focus in front of the lens, and being refracted parallel to the principal axis (this cannot be drawn if the object is closer to the lens than the principal focus).
If a full lens is drawn, refraction is represented from the centre of the lens and not from each of the boundaries.
The image forms where the rays meet. It is represented by another arrow (labelled ‘Image’). If the image is virtual (because the object is nearer the lens than the principal focus), the diverging rays must be traced back in straight lines until they meet.
What does the shutter in the camera do?
Allows the user to adjust the length of time that light enters the camera, which controls the amount of light to which the photosensitive surface is exposed
How do you describe the nature of images?
magnified or diminished (how big the image is compared to the object)
upright (the same way up as the object) or inverted (upside down compared to the object) real or virtual (whether it can be projected on a screen or not)
What images do diverging lenses always produce?
Diverging lenses always produce images that are virtual, upright and diminished (smaller) in size compared to the object. The image always appears to come from the same side of the lens as the object.
What is accommodation?
The eye can alter the shape and curvature of the lens to adjust the degree of refraction. This is called accommodation. It allows light to be focused onto the retina from near or distant objects.
What does the focusing screw in the camera do?
Allows the user to adjust the focus for nearer or more distant objects.
What do the suspensory ligaments do?
Slacken or stretch as the ciliary muscles contract or relax, to adjust the thickness and curvature of the lens.
What does the lens do?
Refracts light to focus it onto the retina. The amount of refraction can be adjusted by altering the thickness and curvature of the lens.
What do the ciliary muscles do?
Adjust the shape of the lens to make it more or less curved, so as to increase or decrease the refraction of light.
How do you find the image produced on a ray diagram for a diverging lens?
As the image is virtual the rays leaving the lens must be traced backwards in straight lines until they reach a point at which they cross. This is where the virtual image appears to come from.
What does the iris do?
Controls how much light enters the pupil.
What are optical fibres?
An optical fibre is a thin rod of high-quality glass. Light getting in at one end undergoes repeated total internal reflection - even when the fibre is bent - and emerges at the other end.
What happens when light rays hit a diverging lens?
Diverging lenses refract the parallel rays of light so that they spread apart from one another. This means that they form a virtual image - an image that cannot be projected onto a screen. The diverging rays can only be seen by the eye, and appear as having come from a different point to where the object is. The point at which the rays appear to have come from is the principal focus (F). The focal length is the distance between the centre of the lens and the virtual image.
What is the focal length of a lens determined by?
the refractive index of the material that the lens is made from and the curvature of the two surfaces of the lens (how thick or fat the lens is)
How are X-rays used to produce medical imaging?
In older X-ray machines, white photographic film is placed behind the patient. X-rays pass through the patient’s body and into the photographic film. The film turns black where X-rays hit it. Bones absorb (stop) X-rays, so the film stays white where the bones are. Fractures show up as dark areas in the image of the bones on the film.
Why are the ionizing properties of X-rays dangerous?
The ionising properties of X-rays mean that they can damage the DNA in cells. The ionising effect of X-rays is more damaging to actively dividing cells in the body.
What does the pupil do?
Allows light to pass through as it enters the eye.
What are CT scans?
Traditional X-ray imaging gives a two-dimensional (2D) view of the body from one angle. This can result in detail being obscured by other structures in the body. Computerised tomography (CT) scans involve taking a range of X-ray images from various positions.
What does the cornea do?
Refracts light as it enters the eye (by a fixed amount).
What is a converging lens and what is the symbol for it?
A converging lens is curved outwards on both sides. It is represented by A diagrammatical interpretation of a convex lens which is a vertical line with arrow heads at each end pointing away from the vertical.
What precautions are in place to protect patients from the dangers of X-rays?
Patients are limited to the number of X-rays they are allowed to have so their bodies are not exposed to too much radiation. X-ray machines also produce relatively low energy X-rays, which reduce the risk of them damaging human tissue.
What is the far point?
The far point is the furthest an object can be from the eye without it appearing blurred.
What images do cameras produce?
Like the eye, the image produced by a camera is diminished, inverted and real.
What is the range of vision for someone with normal sight?
the near point is 25 cm and their far point is infintity.
What does the retina do?
Contains the light receptors, which trigger electrical impulses to be sent to the brain when light is detected.
What is the power of the lens?
The degree to which a lens refracts the light is the power of the lens, measured in dioptres, D.
What can long-sightedness be caused by?
The eyeball being too short - so the distance between the lens and retina is too small.
A loss of elasticity in the lens - meaning it cannot become fat enough to focus (which is often age-related).
As a result, the lens focuses light behind the retina instead of onto it.
What controls how thin a lens will be?
For a given focal length, the greater the refractive index, the flatter the lens can be. This means that modern lenses made from materials with a high refractive index are thinner, making them lighter for people to wear.
What is the near point?
The near point is the closest an object can be from the eye without the object appearing blurred.
What are the properties of X-rays?
X-rays transfer energy and are transverse waves (their oscillations are perpendicular to the direction of energy transfer).
they have a very short wavelength (about the same size as the diameter of an atom
they cause ionisation (adding or removing electrons in atoms and molecules)
they affect photographic film in the same way as visible light (turning it black)
they are absorbed (stopped) by metal and bone
they are transmitted by (pass through) healthy body tissue
What happens if light hits a medium at 90 degrees?
refraction does not happen if rays of light cross the boundary at 90°. In this case, they carry straight on.
How can short-sightedness be corrected?
Short-sightedness can be corrected by placing a diverging lens in front of the eye
What is the lens?
Transparent, bi-convex, flexible disc behind the iris. It is attached to the ciliary muscles by the suspensory ligaments.
Why are X-rays better than ultrasounds for medical imaging?
Compared to ultrasound, X-rays have a much shorter wavelength (higher frequency). This means that X-ray photographs produce higher quality images than ultrasound scans. They show greater detail and clarity. This is vital in terms of detecting small bone fractures or abnormalities.
What is the retina?
The lining of the back of eye containing two types of light receptor cells. Rods are sensitive to dim light and black and white. Cones are sensitive to colour.
What does it mean if the magnification is 1?
A magnification of 1 shows that the image and object are the same size.
What happens when light ray hit a converging lens?
Rays from a single point on a distant object arrive at the lens parallel to one another. Converging lenses refract these parallel rays so that they are come together at a point called the principal focus (focal point). These lenses focus the rays of light to produce a real image - an image that can be projected onto a screen. The focal length is the distance between the centre of the lens and the image
How can you treat cancer with X-rays?
One method of treating cancer is to direct high energy X-rays at the tumours. This causes so much damage to the cancerous cells that they die. This treatment is called radiotherapy.
Why are Ultrasounds used instead of X-rays for pre-natal screening?
X-rays are ionising which means that X-rays damage living tissue and the DNA within cells. Rapidly dividing cells are particularly vulnerable to the effects of ionising radiation. This makes X-rays particularly damaging to a developing foetus. Ultrasound waves are not ionising, and so they are safe to use when performing a foetal scan.
What structure in the eye is the photosensitive surface similar to?
Retina - which detects light and converts it into electrical impulses which are sent to the brain.
What are the first three steps to drawing a ray diagram for a converging lens?
A principal axis is drawn through the centre of the lens at 90° to it.
The object is represented as an upright arrow (labelled ‘Object’) with its base on the principal axis.
The ray diagrams show the rays of light from one point on the object – they do not show all of the rays of light from every part of the object. These rays are drawn from the tip of the object.
What happens when light passes into a less dense medium?
When light passes into a less dense medium, it speeds up. The rays bend away from the normal. This means that the angle of refraction is greater than the angle of incidence.
What is the pupil?
Hole in the middle of the iris.
What happens when the eye focuses on a distant object.
The ciliary muslces relax, the suspensory ligaments stretch so the muscle tension on the lens is high. This causes the lens to become thin and less curved.
What does it mean if the magnification is below 1?
A magnification below 1 indicates that the image is smaller than the object.
What happens when an ultrasound reaches a crack or boundary and why is this useful?
When ultrasound waves reach a boundary between two media (substances) with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through. A detector placed near the source of the ultrasound waves is able to detect the reflected waves. It can measure the time between an ultrasound wave leaving the source and it reaching the detector. The further away the boundary, the longer the time taken.
What does the aperture in the camera do?
Allows the user to adjust the amount of light entering the camera in different light conditions.
What happens to the size of the image if the object gets closer?
is inverted and real - unless the object is at F or closer
What is the critical angle?
The critical angle is the angle above which total internal reflection occurs
What are cameras?
Cameras are devices that focus light onto a photosensitive surface using a converging lens. They have some similarities to the eye.
What is a CCD and what does it do?
A CCD is a charge-coupled device. Modern X-ray machines use CCDs instead of photographic film. The images are formed electronically, allowing them to be recorded and stored more easily than the images from photographic film.
How are CT scans used?
These are processed by a computer to build a three-dimensional (3D) image. This image can be manipulated in order to see the structures within the body at different layers and from different points of view. This lets a doctor gain a much greater insight into what is wrong with a patient.
How is accommodation achieved?
Accommodation is achieved by the contraction or relaxation of the ciliary muscles, which slacken or stretch the suspensory ligaments.
How can the distance traveled by an ultrasound be calculated?
s = v × t
where:
s = distance in metres, m
v = speed in metres per second, m/s
t = time in seconds, s
If you are finding how far away the boundary is, you need to divide it by 2.Why are ultrasounds useful for medical imaging?
The human body is composed of different tissues such as muscle and skin. Ultrasound directed at the body will be partly reflected at the boundary between these different tissues.
How do you work out the magnification of a magnifying glass?
magnification = image height/ object height
Note that magnification is a number without a unit.
How can ultrasounds be used in industry?
An ultrasound machine can be used to detect cracks or flaws in materials such as metal. This is used in industry for quality control procedures to check manufactured objects, such as railway tracks and oil pipelines, for damage or defects.
How is the power of the lens calculated?
Lens power can be calculated using the following equation
P = 1/f
where: P = lens power in dioptres, D
f = focal length in metres, m
What happens when light passes into a more dense medium?
When light passes into a denser medium, it slows down. The rays bend towards the normal (the normal is at 90° to the boundary between the two media). This means that the angle of refraction is smaller than the angle of incidence.
What is special about the power of the lens for a diverging lens compared to a converging lens?
he power of a diverging lens (used to correct short-sight) is negative, while the power of a converging lens (used to correct long-sight) is positive.
What is total internal reflection?
As the angle of incidence increases, so does the angle of refraction. Beyond a certain angle, called the critical angle, all the waves reflect back into the glass and no refraction occurs. This is known as total internal reflection.
What is the object and image distance measured in?
The object distance is measured in focal lengths : F, 2F (twice the focal length), and so on. The image distance can also be measured in focal lengths.
What does it mean if someone is long-sighted?
Someone with long sight can see distant objects clearly, but their point is further away than 25 cm. This means they cannot focus properly on near objects.
What does it mean if someone is short-sighted?
Someone with short sight can see near objects clearly, but their far point is closer than infinity. This means they cannot focus properly on distant objects.
How are ultrasounds used for pre-natal scanning?
These are widely used in pre-natal scanning to check that a foetus is developing normally and to take measurements of its growth. Computers can combine many ultrasound reflection readings to produce a detailed image from them.
What is the refractive index?
The degree to which a material slows the speed of light (compared to its speed in a vacuum) is its refractive index.
What is refraction?
Refraction is the change of direction of light as it passes from one medium (substance) to another. The two media must have different densities, such as air and glass.
What is the cornea?
Tough, transparent covering over the front part of the eye. Convex in shape.
How can ultrasounds be used to cure kidney stones?
Kidney stones are solid crystals formed from substances found in urine. They can sometimes build up into large stones inside the kidney. These can then pass into the ureter and cause a blockage, accompanied by severe pain. High frequency ultrasound waves focused at a kidney stone cause it to vibrate, breaking it into small enough pieces to pass out in the urine.
What is the iris?
Coloured part of the eye that contains muscles. These relax or contract to adjust the size of the pupil.
What can short-sightedness be caused by?
The eyeball being elongated - so that the distance between the lens and the retina is too great.
The lens being too thick and curved - so that light is focused in front of the retina.
What is one of the main differences between the camera and the eye?
One key difference between a camera and the eye is that a camera does not focus light onto the photosensitive surface by adjusting the shape of the lens. Instead, the focusing screws move the lens forwards or backwards in order to focus the image onto the photosensitive surface.
What does the amount of refraction depend on?
the angle the light hits the boundary between the substances
the difference in relative densities, between the two media (which determines how much the speed of light changes)
What happens to the orientation and the type of the image if the object is more the one focal point away?
is inverted and real - unless the object is at F or closer
What is a diverging lens and how is it represented?
A diverging lens is curved inwards on both sides. It is represented on diagrams by A diagrammatical interpretation of a diverging lens which is a vertical line with arrow heads at each end pointing towards the vertical.
What does it mean if the magnification is above 1?
A magnification above 1 shows that the image is larger than the object.
What does the photosensitive surface in the camera do?
Detects and records the light which is focused onto it. This can be on photographic film, but digital cameras use CCDs which convert light into electrical signals which can be stored.
What precautions are in place to protect hospital staff from the dangers of X-rays?
Hospital staff are also at risk from repeated exposure to low levels of X-rays. Shielded walls containing lead are built into all X-ray rooms to protect people outside the room. They have warning signs to show when the room is in use so that people do not enter.
How do you calculate the critical angle of a material?
refractive index = 1/sin c
where c is the critical angle in degrees, °
So: sin c = 1 ÷ refractive index
How are optical fibres used in endoscopes?
A doctor can insert a bundle of optical fibres into the body. Some carry light into the body, and some carry light reflected off internal body surfaces back out. This allows the doctor to see the inside of the body clearly – and help them diagnose diseases like cancer, or see what they are doing during keyhole surgery.
What are the suspensory ligaments?
Connect the ciliary muscles to the lens and hold the lens in place.
How are lasers used in eye surgery?
Lasers are used in eye surgery to repair damaged retinas and to correct vision defects. A laser can be used to precisely cut the cornea, altering its shape to correct a person’s vision.
What structure in the eye is the lens of a camera similar to?
Lens - which focuses light onto the retina.
What is cauterising?
In medicine, lasers are used to destroy damaged tissue or to stop bleeding, eg after surgery. This is known as cauterising
What precautions are in place to protect radiographers from the dangers of X-rays?
Only trained specialist staff - called radiographers - are allowed to use X-ray machines. They routinely leave the room, or stand behind a screen containing lead, whenever X-ray machines are in use. In situations where radiographers cannot stand behind a screen, they wear lead aprons which act as a protective layer of clothing.
What happens when the eye focuses on a near object?
The ciliary muscles contract, the suspensory ligaments slacken so the muscle tension on lens is low. This causes the lens to become fat and more curved.
What structure in the eye is the focusing screw similar to?
Ciliary muscles - which stretch or slacken the suspensory ligaments to adjust the shape of the lens
What are the ciliary muscles?
Muscles connected to the lens by suspensory ligaments.
What are the uses of lasers?
cutting through materials (such as metal)
burning (eg laser engraving) cauterising
What happens if the object is placed between the focal point and the lens?
If the object is placed between the principal focus (F) and the lens, a converging lens will produce a magnified, upright, virtual image. The image can only be seen by looking through the lens, and it appears on the same side of the lens as the object.
How can long-sightedness be corrected?
Long-sightedness is corrected by putting a converging lens in front of the eye, as shown in the diagrams below.
What does the optic nerve do?
The optic nerve carries impulses from the retina to the brain.
What does the lens in the camera do?
To focus light onto the photosensitive surface at the back of the camera. This can either be photographic film or a CCD (charge-coupled device).
What are the advantages of CT scans over traditional X-rays?
CT scans provide even higher quality images than normal X-ray photographs. The 3D images produced by CT scans allow analysis of different levels through the body without other structures obscuring the area of interest (as can happen with traditional X-ray imaging).
What structure in the eye is the aperture similar to?
Iris - which adjusts the amount of light entering the eye through the pupil.
What are the uses of optical fibres?
Optical fibres have become very important in high-speed communications, such as cable TV and high-speed broadband services. Information, in the form of pulses of light, is sent down bundles of optical fibres. Fibre optic cables are able to carry more signals than traditional copper cable telephone lines.
What happens to the position of the object as the object gets closer?
gets further away as the object gets closer - unless the object is at F or closer
What is a laser?
A laser produces an intense narrow beam of light. Laser light has a low divergence - which means that it spreads out very little.
How do you calculate the refractive index?
refractive index = sin i/sin r
Where:
i is the angle of incidence
r is the angle of refraction
How do you draw a ray diagram for a diverging lens?
Ray diagrams for diverging lenses follow similar conventions to those for converging lenses, but how the location of the image is determined is slightly different.
Three rays are drawn:
A ray passing parallel to the principal axis and being refracted away from the principal axis.
A ray directed at the principal focus behind the lens - but being refracted by the lens so that it runs parallel to the principal focus after leaving the lens.
A ray passing in a straight line through the centre of lens without being refracted.
What are the last three steps to drawing a ray diagram for a converging lens?
Not all rays from one point are shown. For simplicity, only three are shown:
a) A ray passing parallel to the principal axis and being refracted to a principal focus behind the lens.
b) A ray passing in a straight line through the centre of the lens without being refracted.
c) A ray passing through the principal focus in front of the lens, and being refracted parallel to the principal axis (this cannot be drawn if the object is closer to the lens than the principal focus).
If a full lens is drawn, refraction is represented from the centre of the lens and not from each of the boundaries.
The image forms where the rays meet. It is represented by another arrow (labelled ‘Image’). If the image is virtual (because the object is nearer the lens than the principal focus), the diverging rays must be traced back in straight lines until they meet.
What does the shutter in the camera do?
Allows the user to adjust the length of time that light enters the camera, which controls the amount of light to which the photosensitive surface is exposed
How do you describe the nature of images?
magnified or diminished (how big the image is compared to the object)
upright (the same way up as the object) or inverted (upside down compared to the object) real or virtual (whether it can be projected on a screen or not)
What images do diverging lenses always produce?
Diverging lenses always produce images that are virtual, upright and diminished (smaller) in size compared to the object. The image always appears to come from the same side of the lens as the object.
What is accommodation?
The eye can alter the shape and curvature of the lens to adjust the degree of refraction. This is called accommodation. It allows light to be focused onto the retina from near or distant objects.
What does the focusing screw in the camera do?
Allows the user to adjust the focus for nearer or more distant objects.
What do the suspensory ligaments do?
Slacken or stretch as the ciliary muscles contract or relax, to adjust the thickness and curvature of the lens.
What does the lens do?
Refracts light to focus it onto the retina. The amount of refraction can be adjusted by altering the thickness and curvature of the lens.
What do the ciliary muscles do?
Adjust the shape of the lens to make it more or less curved, so as to increase or decrease the refraction of light.
How do you find the image produced on a ray diagram for a diverging lens?
As the image is virtual the rays leaving the lens must be traced backwards in straight lines until they reach a point at which they cross. This is where the virtual image appears to come from.
What does the iris do?
Controls how much light enters the pupil.
What are optical fibres?
An optical fibre is a thin rod of high-quality glass. Light getting in at one end undergoes repeated total internal reflection - even when the fibre is bent - and emerges at the other end.
What happens when light rays hit a diverging lens?
Diverging lenses refract the parallel rays of light so that they spread apart from one another. This means that they form a virtual image - an image that cannot be projected onto a screen. The diverging rays can only be seen by the eye, and appear as having come from a different point to where the object is. The point at which the rays appear to have come from is the principal focus (F). The focal length is the distance between the centre of the lens and the virtual image.
What is the focal length of a lens determined by?
the refractive index of the material that the lens is made from and the curvature of the two surfaces of the lens (how thick or fat the lens is)
How are X-rays used to produce medical imaging?
In older X-ray machines, white photographic film is placed behind the patient. X-rays pass through the patient’s body and into the photographic film. The film turns black where X-rays hit it. Bones absorb (stop) X-rays, so the film stays white where the bones are. Fractures show up as dark areas in the image of the bones on the film.
Why are the ionizing properties of X-rays dangerous?
The ionising properties of X-rays mean that they can damage the DNA in cells. The ionising effect of X-rays is more damaging to actively dividing cells in the body.
What does the pupil do?
Allows light to pass through as it enters the eye.
What are CT scans?
Traditional X-ray imaging gives a two-dimensional (2D) view of the body from one angle. This can result in detail being obscured by other structures in the body. Computerised tomography (CT) scans involve taking a range of X-ray images from various positions.
What does the cornea do?
Refracts light as it enters the eye (by a fixed amount).
What is a converging lens and what is the symbol for it?
A converging lens is curved outwards on both sides. It is represented by A diagrammatical interpretation of a convex lens which is a vertical line with arrow heads at each end pointing away from the vertical.
What precautions are in place to protect patients from the dangers of X-rays?
Patients are limited to the number of X-rays they are allowed to have so their bodies are not exposed to too much radiation. X-ray machines also produce relatively low energy X-rays, which reduce the risk of them damaging human tissue.
What is the far point?
The far point is the furthest an object can be from the eye without it appearing blurred.
What images do cameras produce?
Like the eye, the image produced by a camera is diminished, inverted and real.
What is the range of vision for someone with normal sight?
the near point is 25 cm and their far point is infintity.
What does the retina do?
Contains the light receptors, which trigger electrical impulses to be sent to the brain when light is detected.
What is the power of the lens?
The degree to which a lens refracts the light is the power of the lens, measured in dioptres, D.
What can long-sightedness be caused by?
The eyeball being too short - so the distance between the lens and retina is too small.
A loss of elasticity in the lens - meaning it cannot become fat enough to focus (which is often age-related).
As a result, the lens focuses light behind the retina instead of onto it.
What controls how thin a lens will be?
For a given focal length, the greater the refractive index, the flatter the lens can be. This means that modern lenses made from materials with a high refractive index are thinner, making them lighter for people to wear.
What is the near point?
The near point is the closest an object can be from the eye without the object appearing blurred.
What are the properties of X-rays?
X-rays transfer energy and are transverse waves (their oscillations are perpendicular to the direction of energy transfer).
they have a very short wavelength (about the same size as the diameter of an atom
they cause ionisation (adding or removing electrons in atoms and molecules)
they affect photographic film in the same way as visible light (turning it black)
they are absorbed (stopped) by metal and bone
they are transmitted by (pass through) healthy body tissue
What happens if light hits a medium at 90 degrees?
refraction does not happen if rays of light cross the boundary at 90°. In this case, they carry straight on.
How can short-sightedness be corrected?
Short-sightedness can be corrected by placing a diverging lens in front of the eye
What is the lens?
Transparent, bi-convex, flexible disc behind the iris. It is attached to the ciliary muscles by the suspensory ligaments.
Why are X-rays better than ultrasounds for medical imaging?
Compared to ultrasound, X-rays have a much shorter wavelength (higher frequency). This means that X-ray photographs produce higher quality images than ultrasound scans. They show greater detail and clarity. This is vital in terms of detecting small bone fractures or abnormalities.
What is the retina?
The lining of the back of eye containing two types of light receptor cells. Rods are sensitive to dim light and black and white. Cones are sensitive to colour.
What does it mean if the magnification is 1?
A magnification of 1 shows that the image and object are the same size.
What happens when light ray hit a converging lens?
Rays from a single point on a distant object arrive at the lens parallel to one another. Converging lenses refract these parallel rays so that they are come together at a point called the principal focus (focal point). These lenses focus the rays of light to produce a real image - an image that can be projected onto a screen. The focal length is the distance between the centre of the lens and the image
How can you treat cancer with X-rays?
One method of treating cancer is to direct high energy X-rays at the tumours. This causes so much damage to the cancerous cells that they die. This treatment is called radiotherapy.
Why are Ultrasounds used instead of X-rays for pre-natal screening?
X-rays are ionising which means that X-rays damage living tissue and the DNA within cells. Rapidly dividing cells are particularly vulnerable to the effects of ionising radiation. This makes X-rays particularly damaging to a developing foetus. Ultrasound waves are not ionising, and so they are safe to use when performing a foetal scan.
