3. Waves Flashcards
Waves
Oscillations in a medium that transfer energy not matter. Examples are water waves, electromagnetic waves and sound waves.
Can be shown as a series of wavefronts moving away from the source of vibration, transferring energy
Peaks
High points / Crests
Troughs
Low points
Medium
The material through which a wave passes. The plural of medium is ‘media’. Waves travel at different speeds in different media.
Electromagnetic waves
Transerve waves that are vibrations in electric and magnetic fields. These are the waves that transfer energy across the Universe. Electromagnetic waves of different wavelengths make up the electromagnetic spectrum.
Wavefront
A line showing where all the points of the wave are being disturbed in the same way – for example, where all of the crests (peaks) of the waves are at a particular time.
Wavefronts are parallel to the propagation of the wave (direction the waves are travelling)
Direction of propagation
The direction that a wave travels in, this is the direction that energy is transferred.
Transverse wave
The particle oscillates perpendicular (at right angles) to the direction of propagation of the wave
(traditional wave form)
(Particle stays the same length from the source of the wave. It only oscillates (moves) up an down, forming a wave)
Longitudinal waves
The oscillation of the particles is parallel (in the same direction) to the direction of propagation of the wave.
(Particles move back and forth but always end up back where they began. eg. to the right, then to the left etc.)
Wave made of compressions and rarefactions.
Compression - Particles are closer together than normal due to a sound wave passing through it.
Rarefaction - Particles are further apart than normal due to a sound wave passing through it.
(Drawn as a transverse wave where compression = peak and rarefaction = trough)
Examples of transverse waves
- Ripples along water surface –> water moves up and down but wave motion is outwards, along water surface.
- Waves formind in a shaken rope –> wave moves along the length of the rope while the vibrations are from side to side (or up and down)
- Visible light + other electromagnetic radiation –> However, no vibrating particles. Instead there are vibrating electric and magnetic fields –> Vibration in these fields = perpendicular to the derection the radiation travels.
Wave speed (info)
- Measured in m/s
- Moves at the same speed throughout a medium (doesn’t differ between peaks and troughs)
Frequency of a wave
- Number of complete waves (1 wavelength –> from peak to peak/trough to trough) that pass a point each second
- Frequency measured with a unit called hertz (Hz)
- 1 Hz = one complete vibration per second
- 1 kHz (kilohertz) = 1000 Hz
Hertz
The unit of measurement for frequency. Symbol Hz. 1 Hz equals one complete wave/oscillation passes per second.
Wavelength
- The distance between one wave crest (peak) and the next.
- Or the distance between one trough and the next.
- As wavelength is a distance, it is measured in metres (m).
Amplitude
- The distance between the wave crest (peak) and the centre of the wave
- This is the greatest distance a particle moves from its rest position.
- The higher the amplitude a wave has, the more energy it transfers as it moves.
Wave speed (formula)
wave speed (m/s) = frequency (Hz) × wavelength (m)
v = f λ
λ = lamba (greek letter) –> represents wavelength in the formula.
Speed of a wave through a certain medium is fixed. All waves have the same speed through air.
Earthquake waves
Earthquakes are caused by sudden movement of the Earth’s crust. Enormous forces build up as continental plates push against each other and large amounts of energy are released as the pressure becomes too much and the plates slip past each other. These events cause two different types of a waves:
Primary wave and Secondary wave
Primary wave (earthquakes)
Primary waves are longitudinal waves. The ground is compressed in the same direction as the wave travels.
This can shake buildings from side to side as the wave passes through the ground beneath them.
Secondary waves (earthquakes)
Secondary waves are transverse waves. The ground rises and falls as the wave passes through it.
This will shake buildings up and down.
Reflection
A change in direction of a wave as it reaches a boundary.
‘Wave bounces back off the boundary’
Refraction
When a wave changes direction due to it changing speed when it moves from one material to another.
Refraction causes a change in direction unless the light enters along the normal.
Diffraction
The spreading of a wave as it moves through a gap or past the edge of an obstacle.
Reflection - Incident wave / wavefront
The wave / wavefront before it reaches the boundary
Reflection - Reflected wave / wavefront
Wave / wavefront after it has hit the boundary and bounced off.
- If the wavefront is parallel to the surface when it hits, the wavefront will be reflected back in the direction it came from.
- If the wavefront hits the surface at an angle, then its direction of travel will change.
- During reflection, the wave speed, frequency and wavelength does not change. Only the direction in which the wavefront is travelling changes.
Diffraction - Complex explanation
- Diffraction affects all waves
Diffraction through a gap….
- The effect is small unless the size of the gap and the wavelength of the wave are of a similar size.
- Why we don’t see light waves diffracting as they pass through a doorway –> Wavelength of the light is much smaller than the size of the gap.
- Soun waves have a wavelength of approx a few cm to a metre –> enough to significantly diffract through a doorway –> Why we can hear people approaching.
- Light waves can be diffracted by passing light through gaps a fraction of a mm. –> light sreads with wave fronts becoming circular and spreading out from the gap.
Diffraction around the edge of an object…
- Diffraction also happens when waves pass by the edge of an object
- The waves spread around the edge and can travel behind it.
- This amount of diffraction increases as the wavelength of the wave increases.
- Waves with long wavelengths diffract more than waves with short wavelengths.
- Sound waves have much longer wavelength than light waves –> much more strongly diffracted by edges.
- Allows sound waves to spread around corners in a way that is not possible for light.
Electromagnetic waves (complex)
- Transverse waves with unusual properties not typical to transverse.
- Can travel through a vacuum (don’t rely on vibration of any kind of particle to travel)
- Electromagnetic waves are vibrations in linked electric and magnetic fields moving through space.
- All electromagnetic waves travel at the same very high speed in a vacuum – much faster than any other type of wave.
Interaction between electric and magnetic fields to form electromagnetic waves
(READ EXTRA ON)
- Electromagnetic waves do not have oscillating particles – they have oscillating fields.
- Electromagnetic waves have a wide range of wavelengths and frequencies.
- There are two linked oscillating fields which are at right angles to each other and at right angles to the direction of propagation.
- In a vacuum the speed of propagation is 3.0 × 10⁸ m/s for all electromagnetic waves.
Speed of propagation of electromagnetic waves in a vacuum
3.0 × 10⁸ m/s
(Fastest anything can possibly travel)
Speed of propagation
Fancy phrase for wave speed
Electromagnetic spectrum
The complete set of electromagnetic waves, including radio waves, microwaves, infrared radiation, visible light, ultraviolet, X-rays and gamma rays.
Lowest frequency / Longest wavelengths
Radio waves
Microwaves
Infrared radiation
Visible light
Ultraviolet
X-rays
Gamma rays.
Highest frequency / Shortest wavelengths
Electromagnetic spectrum - Ionising radiation
Ultraviolet
X-rays
Gamma rays
Electromagnetic spectrum - Non-ionising radiation
Radio waves
Microwaves
Infrared radiation
Visible light
Radio Waves
A type of transverse wave that consists of vibrating electric and magnetic fields.
Radio waves have the longest wavelength of the electromagnetic waves.
They are produced when electrons are moved back and forth inside wires by varying electric currents.
Different wavelengths are produced by making the electrons oscillate at different frequencies.
Radio waves - TV / Radio Signals (Uses)
TV / Radio signals
- Used to transmit TV and radio signals.
- Transmitted from large radio transmission towers (Can produce powerful radio signals so detected over great distances)
- Transmission type = terrestrial broadcasting
- Most radio signals = Refracted (ASK MR. GARDINER) back by
upper atmosphere (thus can travel past curvature of the earth to reach distant places) - TV signals = shorter wavelength radio waves –> don’t refract as much –> escape the atmosphere.
Radio waves - Radio Astronomy
- Many objects emit radio waves that can pass through dust clouds in space + atmosphere –> can be detected on earth.
- Exceptionally large radio wave detectors designed to detect weak radio signals coming from stars and black holes –> Detect radio waves.
- Helpful bc not all objects in space emit visible radiation + dust/ clouds of dust obstruct view.
Radio waves - Radio frequency identification (RFID)
Radio frequency identification (RFID)
- Small reciever, containing a tiny computer chip + aerial is attatched to an object (with unique ID no. + other info in it)
- Transmitter send out radio waves + reciever absorbs them.
- Radio wave provides enough power for the RFID chip to send back a signal containing its data.
- Contactless payments using credit cards use RFID chips.
- When card is placed near a pay point –> it sends out a signal (requesting details in the form of a radio pulse eg. How much to pay and who is requesting the payment.)
- The absorbed radiation powers the RFID chip on the card.
- The credit card sends back the details of the user.
- The pay point checks the details, usually in a fraction of a second, and accepts payment.
- Mobile phones also commonly have active RFID chips, which can send out requests of their own.
Microwaves
Electromagnetic waves are a type of electromagnetic wave.
They are similar to radio waves and are also produced by oscillating electrons in wires. However, they have a shorter wavelength, usually a few centimetres or millimetres.
Microwaves - Microwave oven
- Microwaves are absorbed strongly by some of the molecules present in food and living tissue, causing a heating effect.
- This effect is used to cook food in microwave ovens.
- Microwaves are produced by a small transmitter inside the oven.
- They are absorbed by the food which heats up rapidly.
- The oven is made of metal so that the microwaves reflect back inside the chamber and heat the food instead of escaping.
Microwaves - Satellite and phone communication
- Microwaves = used to transmit signals to satellites in orbit around the Earth.
- Many radio waves cannot penetrate the upper atmosphere easily; however, microwaves can and so make satellite communication possible.
- Low-power microwaves are used in mobile phone networks.
- The mobile phones produce weak microwave signals which are detected by dishes on towers throughout towns and cities
- The towers send signals back to the mobile phones.
- The aerials needed to transmit and receive microwaves are much smaller than those needed for radio waves.
- A mobile phone will have a microwave receiver which is only a few centimetres long.
- Sometimes, if you hold your phone the wrong way or put it inside a badly designed case, you can block this receiver and make it difficult for the phone to receive a signal.
- Many houses and businesses have wireless local area networks (LANs) connected to the internet.
- These use microwaves to relay signals between devices, like laptops or smart TVs, and a ‘router’ usually connected to a wired phone network.
- The microwaves allow communications through the air and can even pass through walls, although this will weaken the signal.
- Mobile phone signals can also pass through some walls; this also reduces their signal strength so you may lose reception indoors.
Microwaves - Dangers
- Heating effect caused by microwaves = dangerous
- This could easily damage living tissue.
- As mobile phones use microwaves, some people were concerned that they would cause brain damage as the phones were placed next to ears, close to our brains.
- While the brain will absorb some of the radiation, the power level of the phone is far too low to produce a damaging effect.
Infrared radiation
Electromagnetic waves used for cooking (grilling) and in optical fibres for communications. Emitted by all objects but more radiation is emitted by hotter objects.
Infrared - Remote Controls
- Remote controls for TV use infrared radiation.
- To change channel/ volume, a diode on the remote control emits pulses of infrared radiation which is detected by a sensor on the television set.
- The radiation can travel a few metres through the air before being completely absorbed, and so the control has a limited range –> remote also has to be pointed directly at TV.
- Some television sets now operate on Bluetooth radio signals.
- These signals can pass through walls and so you could operate a TV, or music player, from another room.
- They have a fairly short range, up to 50 m + signal is weakened as it passes through walls –> but powerful enough for most houses.
Infrared - Cooking
- Infrared radiation has a heating effect when it is absorbed.
- An electric grill, or toaster, heats metal wires to very high temperatures so they emit large amounts of radiation
- Absorbed by surface of food palced nearby + cooks it
- Similar effect achieved with flames (emit large amounts of infrared.
Infrared - Thermal imaging + alarms
- As the temperature of an object increases, the amount of infrared radiation it emits increases too
- This effect is used in thermal imaging.
- Infrared cameras can produce images of hot or warm objects, even when there is no visible light.
Uses include…
- Night vision
- Identifying energy losses from houses due to poor insulation
- Finding hotspots indicating faults in electronic equipment
- Medical diagnosis.
- Some burglar alarms use simple infrared systems where only temperature change needs to be detected.
- When a burglar moves past an infrared sensor, it detects the radiation they are emitting and sets off the alarm system.
Infrared - Communications system
- Infrared radiation cannot travel far through air without being absorbed –> not of much use in transmitting information through the atmosphere.
- Can however travel very large distances though materials like glass –> often used in fibre optic networks to transmit information.
Infrared - Dangers
- Infrared radiation has a heating effect when it is absorbed –> an cause skin burns. (felt when standing near fire)
- The closer you are to the source, the more infrared radiation you will absorb and the more likely you are to get burned.
- Firefighters sometimes wear silver-coloured suits to prevent burns from working near intensely hot objects.
- The silver surface reflects the radiation instead of absorbing it.
Visible light
Electromagnetic waves with a wavelength that we can see.
- Used in any optical instrument where we want to see objects directly, such as microscopes, telescopes and binoculars.
- It is also used in photography where the light is detected by sensors in the camera or by photographic film.
Ultraviolet radiation
- Ultraviolet radiation (UV) is produced by the sun and some fluorescent tubes.
- Most of the UV radiation emitted by the sun is absorbed by our atmosphere before it reaches the surface of the Earth.
- Some parts of the UV spectrum can pass through.
UV - Clothes/ Make-up
- Some chemicals can absorb UV radiation and then emit visible light instead.
- Clothing which has been washed in biological washing powder is a good example.
- When the clothes are exposed to UV light, they appear to glow and this effect is used in fairgrounds and discos
- Some clothes are designed using UV-sensitive dyes to increase this effect.
UV - Money
- Paper produced from wood pulp is bleached to make it very white.
- This bleaching process causes the paper to glow when exposed to ultraviolet light.
- Most paper banknotes are produced from unbleached cotton which does not glow when exposed to UV
- Fake banknotes will glow under an UV lamp.
- Many new notes are made from plastics with some sections designed to glow in UV and other sections designed not to glow, depending on which inks have been used to dye the plastic.
UV - Water filtration
- Ultraviolet light is also used to sterilise water in waste treatment plants and water supplies.
- Water can contain harmful microbes, especially wastewater, even after filtering.
- To kill these microbes, the water is exposed to high intensity UV radiation.
- This is absorbed by the microbes and kills them, making the water much safer to drink or release into the environment.
Dangers of UV radiation
- Ultraviolet light is damaging to the skin and eyes.
- Long, or intense, exposure to the skin can cause burns and, if the skin cells are damaged, lead to skin cancers.
- The retina at the back of the eye is particularly sensitive and, because we cannot actually tell if something is emitting UV, it is easy to damage the retina during sun tanning.
X-rays
High energy electromagnetic waves which can pass through some materials but not others. Used to take pictures of bones, teeth for example. Can be harmful.
- Produced when fast-moving electrons are brought to a sudden stop inside an X-ray tube.
- They are also emitted by stars and other astronomical objects but the Earth’s atmosphere prevents most of this radiation from reaching the Earth’s surface.
X-ray - Uses
- X-rays can pass through some materials easily, such as muscle, but are absorbed by others, like bone.
- This allows us to use X-ray machines to take photographs of the bones inside our body.
- A photographic plate (or X-ray camera) is placed behind the patient’s body and then the patient is exposed to a burst of X-rays.
- The areas where there is only muscle or skin allow the X-rays to pass through, but the areas with bone block some of the X-rays.
- These X-rays are detected by the photographic film, making the areas with soft tissue dark but leaving the areas with bone white.
- Doctors can then examine the images to find breaks in bone.
- Similar process used to scan bags in security at airports.
Dangers of X-ray
- X-rays can damage cells (especially growing cells) and so are not used unless they are really needed.
- They are best avoided during pregnancy.
- The medical staff who operate X-ray machinery protect themselves from repeated exposure by leaving the room while X-rays are taken or by standing behind lead sheets.
Gamma rays (EM spectrum)
- Similar properties to X-ray but different origin-
- Gamma rays are produced during the radioactive decay of a nucleus.
- To produce strong gamma rays, samples of radioactive material, such as strontium-90, are collected together inside a lead-shielded container.
- The lead absorbs most of the gamma rays emitted by the radioactive material; a small window in the container is opened when the gamma rays are needed.
(Shortest wavelength + highest frequency)
Gamma rays - Sterilisation
- Gamma radiation can also be used to sterilise medical equipment.
- The equipment is cleaned and placed inside sealed containers which are then exposed to high doses of the radiation. T
- he gamma rays penetrate the containers and kill any bacteria or viruses inside, leaving sterilised equipment.
- Similar techniques can be used to kill bacteria on food before shipping or storage.
Gamma rays - Gamma imaging
- As gamma radiation is similar to X-rays, it can pass through the body and then be detected by a device known as a gamma camera .
- This process uses ‘gamma imaging’ where a patient is injected with a radioactive substance which emits gamma radiation.
- The substance is designed to collect in specific organs, such as the thyroid gland, and the amount of radiation emitted indicates how heathy the organ is.
Gamma rays - Radiotherapy
- Beams of gamma rays are used to kill cancerous cells in a procedure called radiotherapy .
- Gamma radiation can kill living cells and also damage the DNA inside the cells, leading to cancer.
- As cancer cells are constantly reproducing, they are highly susceptible to damage from gamma radiation and so a beam is directed at them from a radioactive source.
- The beam is moved around the body, always pointing at the cancerous cells to reduce the amount of radiation the healthy cells are exposed to.
- Gamma radiation damages the DNA inside cells, and this is how it kills cancerous cells, but it also damages the DNA in healthy cells.
- This can cause cancer, so radiotherapy needs to be carefully controlled so that it does more good than the harm it causes.
Gamma rays - Uses (list)
- Radiotherapy
- Gamma imaging
- Sterilisation
Infrared radiation - Uses (list)
- Remote controls
- Cooking (eg. toasters)
- Thermal imaging and alarms
- Fibre optic networks (communication system)
UV - Uses (list)
- Clothes/make-up
- Detecting fraudulent banknotes
Microwaves - Uses (list)
- Microwave ovens
- Sattelites + communications
Radiowaves - Uses (list)
- Radio and television broadcasts
- Radio astronomy
- Radio frequency identification (RFID)
Optical fibres - (ADD TO)
- Total internal reflection allows us to direct light to travel in any path we want.
- This effect is used in optical fibres. These are very thin glass or plastic tubes (glass fibres) which can bend.
- Many communications or computer networks use optical fibres
- A signal can be transmitted all the way around the Earth in a fraction of a second through an efficient fibre optic network (stops lagging)
- Optical fibres use either visible light or infrared radiation to transmit very large amounts of information very quickly.
How it works
- A short pulse of light is produced at one end of the fibre.
- Bc of total internal reflection, the pulse (light) travels along the fibre and, if it reaches a boundary, it reflects back into the fibres core –> the light doesn’t leave the fibre. (stays in glass fibre even when it curves)
(The shape of the fibre ensure the angle of incidence is always greater than the critical angle)
Two types of sattelites
- Low Earth Orbit satellites
- Geostationary satellite
Geostationary satellite
A satellite positioned so that it stays in a fixed position in the sky as the Earth rotates. It is about 36 000 km above the Earth’s surface.
- Takes them exactly one day to complete one orbit around the planet
- (Since the earth also takes one day to rotate, the sattelite always stays in the same position above the earth)
- This allows satellite dishes to remain pointed at the same position in the sky to receive a signal.
Low Earth Orbit (LEO) satellites
A satellite positioned close to the surface of the Earth, up to 100 km –> moving quickly across the sky.
- Some of these LEO satellites are used for spying or observing weather.
- Many are now part of phone networks or the internet.
- Signals from satellite phones or internet devices are received by the satellites + passed between them, returning to the ground, sometimes on the opposite side of the Earth, within a small fraction of a second.
Reflection in a plane mirror - How the image will look
- If the mirror is plane, the image won’t get distorted
- The object will also be the same size + the image will seem to be the same distance behind the mirror as the object is in fron of the mirror.
(Virtual image)
Incident ray
The path of a beam of light which travels from source and reaches a mirror.
Reflected ray
The path of a beam of light which has reflected from a mirror.
Angle of incidence
The angle at which a wave or ray approaches a boundary or surface. Measured from the normal. (i)
(The normal is a imaginary perpendicular line from the mirror, against which angles of incididence/reflection are measured)
Angle of reflection
The angle at which a wave or ray leaves a boundary or surface after being reflected. Measured from the normal. (r)
(The normal is a imaginary perpendicular line from the mirror, against which angles of incididence/reflection are measured)
Law of reflection
Angle of incidence = Angle of reflection
i = r
(Rule applies to ALL reflected waves. Even applies to curved mirrors/ reflecting surfaces)
Virtual image
An image formed by a lens which cannot be projected onto a screen as the rays of light only appear to pass through it.
Virtual images are formed by mirrors and some
lenses. (Image looks like it lives ‘inside’ the mirror)
Real image
An image formed by a lens which can be projected onto a screen. Rays of light pass through the points on a real image.
(Image projected will always be inverted (upside down))
Refraction of light - In glass
- When light moves from air into glass, the light will slow down. This causes it to change direction TOWARDS the normal.
- When light moves from a glass block into air, then it will speed up. This causes a change in direction AWAY from the normal.
- If the ray enters or leaves the block normal to the surface, then there is a change of speed, but the ray continues to travel in the same direction.
The larger the angle of incidence, the greater the change in angle of the rays.
Refractive index
(n) A measurement of the speed of light in a material.
n = speed of light in vacuum / speed of light in a substance
Speed of light in vacuum = (3.00 × 108 m/s)
or
n = Sin (Angle of incidence) / Sin (Angle of reflection)
Internal reflection
When light moves across a boundary between materials some of it is reflected back from the boundary.
Eg.
- A beam of light leaves a glass block and enters the air.
- It refracts away from the normal due to an increase in its speed.
- During this refraction, a small amount of the light reflects off the back surface. This is called internal reflection.
Total internal reflection
When light is perfectly reflected as its reaches a boundary. (Instead of refracting into the second medium) The effect only happens if the angle of incidence is above the critical angle.
Critical angle
The largest angle of incidence which allows light to leave a material. Above this angle the light will be totally internally reflected.
Critical angle formala
n (Refractive index) = 1/ Sin (critical angle)
sin(c) = 1/n
Optical fibres - Uses
- Optical fibres are used in an endoscope.
- Allows doctors to see deep inside the body.
- Endoscopes have several optical fibres bound together in a bundle so that light can travel into and out of the body.
- White light is sent along one fibre into the body and then reflects off the internal organs and into a bundle of organised fibres back to the outside of the body.
- The external end of the endoscope is connected to a camera so the surgeon can see an image of the internal organs as they operate.
- The endoscope can be inserted down the oesophagus or trachea to see into the stomach or lungs.
- Alternatively, small holes can be cut by the surgeon to see, and operate on, other internal organs.
- Endoscopes have small tools fitted to their ends such as tiny tweezers which can take tissue samples.
- They can also transmit very intense laser light allowing surgeons to make cuts or cauterise inside the body.
- Endoscopes are not only used in medicine.
- Similar devices are used by plumbers to investigate blocked drainpipes or by builders to look under floorboards.
Converging lens
A lens which converges (brings together) rays of light.
(convex shape)
- If the rays of light are PARALLEL to each other when they enter the lens + also parallel to the optical axis –> they will converge at the same place, which is called the principal focus
Optical/Principal axis
A line which passes through the principal focus and its optical centre (centre of the lens)
Principal focus
The point at which parallel rays are brought together by a converging lens. This is sometimes called the focal point.
Focal length
The distance between the centre of a lens and it’s principal focal. Measures along the principal axis.
- Powerful lenses have short focal lengths while weaker lenses have longer focal lengths
- Measured in metres or centimetres
Qualities of an image which can be formed by a converging lens
- Enlarged or diminished
- Upright or inverted
- Real or virtual.
Diverging lens
A lens which diverges (spreads out) rays of light.
Concave shape
- If the rays of light are parallel to each other when they enter the lens + parallel to the optical axis…
- They will all be spread so that they can be traced backwards until they seem to originate from the same place (The principal focus)
- Powerful lenses have short focal lengths while weaker lenses have longer focal lengths.
Images in diverging lenses are always…
- Virtual – the rays do not pass through it and so it cannot be projected onto a screen.
- Diminished – it is always smaller than the original object.
- Upright – the same way up as the original object.
Retina
Layer containing receptor cells that responds to light levels and to light of different colours.
Blind spot (eye)
Part of the retina where the optic nerve enters the eye. As it does not contain receptor cells, it is not sensitive to light.
Optic nerve
Bundle of neurones that carries nerve impulses from the retina to the brain.
Cornea
Transparent outer layer that refracts light, focusing it towards the retina.
Iris
The coloured part of the eye that controls the amount of light passing through the pupil.
Pupil
The hole through the iris through which light can pass into the eye from the cornea to the lens.
Lens (eye)
Transparent rounded structure that refracts light, focusing it onto the retina.
Cornea (eye)
A tough, colourless and transparent outer layer covering the iris and pupil of the eye. Most refraction of light happens through the cornea.
Two lenses within the eye
- The outer part of the eye is the cornea . This acts as a converging lens of fixed focal length and produces most of the focusing of light as it enters the eye.
- The lens. This is a converging lens made from a tough, transparent material. Tiny muscles which surround it can be used to stretch it to make it thinner. This increases its focal length and allows us to focus on objects which are further away.
Far point
The most distant point or object your eye can focus on.
Near point
The closest point or object your eye can focus on.
Short-sightedness
- Over time, the lens in your eye may become difficult to stretch and so it cannot be made thin enough by the eye muscles –> results in short-sightedness
- The thick lens causes light for distant objects to be focused in FRONT of the retina so we cannot see them clearly.
- The far point of the eye is no longer at infinity and may only be a few metres away.
- This condition can be corrected by adding a small diverging lens in front of the cornea.
- This can be in the form of spectacles or contact lenses.
- The effect of the lens is to reduce the focusing power of the eye and allow it to focus a clear image onto the retina for distant objects.
Long-sightedness
- Some people develop long-sightedness.
- The lens cannot be made thick enough, or the eyeball is too short.
- The result is that the light from nearby objects is focused on a point behind the retina giving a blurred image.
- The near point becomes further away from the eye, meaning that you might have to hold writing further away to read it easily.
- Long sightedness can be corrected by using a converging lens in front of the eye, increasing its overall focusing power and bringing the near point back to about 25 cm.
Combination - Eye-sight
- Some people are both long-sighted and short sighted
–> complex combinations of lenses can be used in spectacles such as bifocal lenses where the upper half acts as a diverging lens and the lower half as a converging lens
- This allows the user to see distant objects clearly by looking slightly upwards and nearby objects by looking slightly downwards through the different parts of the lens.
Dispersion of light - Why it happens
- All electromagnetic radiation travels at the same speed in a vacuum, but not if the radiation is moving through another material.
- Each different wavelength (or frequency) will travel at a different speed.
- A beam of white light is made up from a range of different frequencies of light (the visible spectrum).
- When white light enters a block of glass, each of the different parts of the spectrum will slow down to a different speed.
–> This means that each colour refracts differently.
- If the ray does not enter along the normal, each of the colours will refract by a different amount.
- Causes the white light to split into its component colours –> known as dispersion.
- When the beam leaves the block, each colour refracts by a different amount again and the colours spread out more
How light disperses
- Red light changes speed the least and so is refracted the least, while violet light has greater changes in speed and so is refracted the most.
- The dispersion of white light produces seven different colours which, (in order of increasing frequency), are: red, orange, yellow, green, blue, indigo and violet.
R - Richard (red)
O - Of (orange)
Y - York (yellow)
G - Gave (green)
B - Battle (blue)
I - In (indigo)
V - Vain (violet)
When sunlight passes through raindrops, the white light can be dispersed and form a rainbow.
Visible light (wavelength/frequency)
Longest wavelength/Lowest frequency
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Shortest wavelength/Highest frequency
Monochromatic light
- White light contains a range of frequencies
- However, some light sources, such as lasers, can produce light which only contains one frequency.
- This kind of light is called monochromatic light.
- Monochromatic light does not disperse when it passes from one medium to another.
Changing the properties of a sound wave
Faster vibrations –> increased frequency –> Increased pitch
Larger vibrations –> Increased amplitude –> Increased loudness
How is a sound wave affected by changes in amplitude and frequency?
- Increasing the amplitude increases the energy transfer and so the sound will be louder.
- Increasing the frequency, increases the pitch.
Oscilloscope
A device used to show wave patterns on screen. When connected to a microphone it can show a representation of sound waves.
Sound waves - Described
Sound waves are a type of longitudinal wave. The particles which form the wave move back and forth in the same direction as the wave travels.
- Wave produces regions where the air particles are compressed closer together (compressions) and regiones where the air particles are spread out more than normal (rarefactions)
- The compressions and rarefactions move away from the source of the sound in all directions, spreading the sound wave outwards.
- The particles don’t actually move, they just vibrate back and forth. The compressions and rarefactions move however.
How we hear sound
- When an object, like the string of a guitar vibrates, it produces a sound wave which travels through the air. (causes air particles to vibrate back and forth)
- The wave travels from the source (the guitar string) through a medium (the air) and reaches a detector (our ear).
- The wave then causes our ear drum to vibrate, and this vibration passes through a series of tiny bones into our inner ear, stimulating nerve cells to send electrical signals to our brain.
- These signals cause the sensation of sound.
Reflection of sound waves
- Sound waves can reflect when they reach a boundary between materials.
- This happens when a sound wave moving through the air reaches a hard surface, such as a wall.
- The wave hits the wall and is reflected away from the wall, back towards its source.
- The reflected sound wave is called an echo.
- Echoes can be a problem in some places, such as concert halls or recording studios, where we do not want reflected sound waves to interfere with the music.
- To reduce the problem, the wall can be coated in soft, sound-absorbing materials.
Echo
The reflection of a sound wave. Sound waves commonly reflect from hard surfaces such as brick walls or cliff faces.
How to estimate the speed of sound
Measure the time taken for sound to travel a long distance –> Distance/time = speed
Cons
- Reaction time may be delayed bc of human error –> to solve this increase distance
To accurately measure the speed of sound, use an electronic stopwatch triggered by sound.
Unit for sound
Hertz (Hz)
Measures the frequency of the wave.
- The higher the frequency, the larger the pitch of the sound wave.
(decible is used to mesure volume of sound)
Speed of sound in different medias
- Sound waves travel at different speeds in different materials.
- Many factors affect this speed, including the density and elasticity of the materials.
- In general, however, sound travels slowest in gases, faster in liquids and fastest of all in solids.
(sound waves cannot travel without a medium (particles) -> no sound in a vacuum.
Ultrasound
Sound waves with a frequency above 20 000 Hz. Humans cannot hear sound waves above this frequency.
Humans can hear between 20 Hz and 20,000 Hz
Ultrasound - Uses (list)
- Measuring distances
- Sonar
- Analysing materials for damage
- Medical scanning
Ultrasound - Measuring distances
- Ultrasound can be used to measure distances using echoes.
- For example, an ultrasound pulse is fired at a wall and the time taken for the wave to reach the wall and return to the transmitter can be used to find the distance the wave has travelled using the relationship:
distance = speed × time
As the speed of sound in air is known, the distance is calculated using the time.
Speed of sound in air
343 m/s
Ultrasound - Sonar
- Ultrasound pulses are used in sonar, a system used to measure distance, such as depth, in the ocean.
- A sound pulse is transmitted from a boat and travels through the water until it reaches an object.
- It is then reflected and some of the reflected sound wave is detected by the boat.
- If the speed of sound in the water is known, then the distance can be calculated.
(The speed of sound in ocean water varies with depth and temperature but is normally around 1500 m/s)
Ultrasound - Analysing materials for damage
- Materials can contains unseen falws
- These flaws can be there when the part is manufactured or can develop over time as it is used.
- If the material broke, it could be disastrous.
- As it is impossible to see inside the parts, inspecting them from the outside might not show the problem.
- Instead, we can send ultrasonic pulses into the materials.
- Any cracks inside would partially reflect the ultrasound, producing detectable echoes.
- Analysis of the echoes will show the size and depth of any flaws in the object.
Ultrasound - Medical scanning
- Ultrasound is commonly used for medical diagnosis and observation.
- An ultrasound pulse is produced and travels into the patient’s body.
- As the pulse moves through different body tissues it is partially reflected and those reflections are detected by a sensor.
- This data is sent to a computer for analysis.
- Sound travels at different speeds in different types of body tissue.
- The computer can use the timings of the echoes and the different speeds of sound to measure the thicknesses of different layers of tissues.
- X-rays are harmful to living tissues because they cause ionisation which damages cells
- That means they should only be used when necessary and they should NOT be used for pregnant women unless unavoidable.
- Ultrasound scanners can produce two or three-dimensional images of tissues using the echo information, safely as ultrasound causes no ionisation.
- This allows them to be used for pre-natal care when doctors want to examine the developing foetus.
