Medical Applications of Physics Flashcards
What are X-rays?
Electromagnetic waves with a high frequency and very short wavelength (about the diameter of an atom!)
What are the properties of X-rays?
- Affect photographic film in the same way as light
- Are absorbed by metal and bone
- Are tranmitted by healthy tissue
X-ray are used to form…
…images on bones on photographic film to check for fractures and dental problems
What does CCD stand for?
Charge-coupled device
What are CCDs?
Charge-coupled devices. They are used to form electronic images of X-rays.
What are CT scanners?
Medical scanners which pick up X-rays and use them to produce digital images of a cross-section through the body or a 3D image on an organ
How can images be taken of some organ made of soft tissue (e.g. the intestines)?
They can be filled with a contrast medium (such as barium sulfate) that absorbs X-rays so they can be seen on an X-ray image.
What do X-rays cause and what effect does this have on the human body?
They cause ionisation which can damage living tissue when they pass through it.
What precautions should be taken when working with X-rays?
- Workers should minimise exposure
- Workers should wear film badges
- Workers should use lead screens to shield them from the X-rays
Explain how X-rays can be used for therapy.
They can be used to treat cancerous tumours at or near the body’s surface.
How are X-rays used in hospitals?
- To make images and CT scans
- To destroy tumours at or near the body surface
What are the links between X-rays and cancer?
They can cause it but they can also be used to treat it
What frequencies of sound can the human ear detect?
Sounds between 20 Hz and 20,000 Hz.
What are ultrasound waves?
Sound waves of a higher frequency than what humans can hear (20,000 Hz)
(The opposite of ultrasound is infrasound - below 20Hz/the range of human hearing)
What can be used to produce ultrasound waves?
Electronic systems
How is ultrasound used to produce an image?
- Electronic systems are used to produce an ultrasound wave.
- When a wave meets a boundary between two different materials, part of the wave is reflected.
- The wave travels back through the material to a detector.
- The time it takes to reach the detector can be used to calculate how far away the boundary is.
- The results may be processed by a computer to give an image
What are s, v, and t in the equation:
s = v X t
- s* = distance, metres
- v* = speed ultrasound wave travels, metres per second
- t* = time taken, seconds
What happens in the time between a transmitter sending out a pulse of ultrasound and it returning to the detector?
The wave has travelled from the trasmitter to a boundary and back - in other words twice the distance to the boundary.
Is ultrasound safer than X-rays and why?
They are safer because they are non-ionising
What are the medical applications of ultrasound?
- Scanning unborn babies
- Scanning soft tissue, such as around the eye
- In therapy, e.g. to shatter kindney stones into smaller pieces so they can be excreted.
What is refraction?
The change of direction of light as it passes from one transparent susbtance to another
What is refractive index?
A measure of how much a substance can refract a light ray
What is the unit for refractive index?
It doesn’t have one (it’s a ratio)
What causes refraction?
Waves change speed when they cross a boundary (enter a different medium). The change in speed causes a change in direction, unless the waves are travelling along the normal.
What happens to a light ray when it crosses from air into glass?
It is refracted towards the normal
Less dense to more dense = towards normal
More dense to less dense = away from normal
When a light ray is refracted when it crosses from glass to air, what else is seen?
A partially refracted ray.
What happens if the angle of incidence in the glass is gradually increased?
The angle of refraction also increases until the refracted ray emerges along the boundary (between the glass and air)
What is the critical angle?
The angle of incidence of a light ray in a transparent substance which produces refraction along the boundary (between it and another medium e.g. air)
What happens if the agle of incidence is increased beyond the critical angle?
The light ray undergoes total internal reflection
When total internal reflection occurs, what is the relationship between the angle of reflection and the angle of incidence?
They are equal
When does total internal reflection occur?
Occurs when the angle of incidence of a light ray in a transparent substance is greater than the critical angle. When this occurs, the angle of reflection is equal to the angle of incidence (this is total internal reflection).
In the equation:
n = 1 / sinc c
What are n and c?
- n* = the refractive index
- c* = the critical angle
What is the unit for refractive index?
There isn’t one, it’s a ratio
What is an endoscope?
A device used to look inside a patient’s body without cutting it open or performing keyhole surgery.
It contains two bundles of very thin optical fibres, one used to transmit light and the other used to see inside the body.
It works by using total internal relfection.
What are optical fibres?
Very thin, glass fibres used to transmit light + light signals using total internal reflection
What may be used as an energy source in an endoscope?
Laser light
As an energy source in an endoscope, what can laser light be used to do?
Carry out surgical procedures such as:
- Cutting
- Cauterising
- Burning
- Laser eye surgery
Why is the colour of the laser light important?
It is matched to the type of tissue to maximise absorbtion
How can eye surgery be carried out?
Can be carried out on the retina using laser light that passes straight through the cornea at the front of the eye but is absorbed by the retina at the back.
In what types of materials does total internal reflection occur?
Only from a more dense material to a less dense material.
What are the two types of lens?
Converging (conves) and diverginf (concave)
What happens to parallel rays of lights that pass through a converging lens?
They are refracted so that they converge at a point called the principal focus
What is the principal focus?
(Focal point)
The point where light rays parallel to the principal axis of a lens are focused (or, in the case of a diverging lens, appear to diverge/originate from)
What is the focal length?
The distance from the centre of the lens to the principal focus
Where is the principal focus of a converging lens (and why)?
On either side because light can pass through the lens in either direction
What happens in a diverging lens if the object is further away from the lens than the principal focus?
An inverted, real image is formed
What is the difference between a real image and a virtual image?
A real image is an image formed where light rays meet.
A virtual image is an image, seen in a lens or mirror, from which light rays appear to come from after being refracted by the lens of refracted by the mirror.
In other words, they don’t actually originate there, it’s the point where they all meet (it’s not a real image).
What symbol is used in ray diagrams to represent a converging lens?
This one
What symbol is used in ray diagrams to represent a diverging lens?
This one
What does the size of the image (real/virtual) depend upon?
The position of the object.
The closer it is to the lens, the larger the image for converging lenses
If an object is nearer to a divergent lens than the prinicpal focus, what happens?
An upright, virtual image is formed behind the object. The image is also magnified - the lens acts as a magnifying glass.
How is magnification calculated?
Magnification=
image height / object height
What happens when parallel light rays pass through a diverging lens?
They are refracted so that they diverge away from a point - this point is called the principal focus
Where is the principal focus of a diverging lens?
On either side because light can travel through the lens in either direction
What type of image is produced by a diverging lens?
Always virtual
What is the difference between diverging and converging lenses?
A converging lens focuses parallel rays to the principal focus.
A diverging lens makes parallel rays spread out as if they came from the principal focus.
When is a real image formed?
By a converging lens if the object is further away from the lens than the principal focus.
When is a virtual image formed?
By a diverging lens.
Or, by a converging lens if the object is nearer to the lens than the principal focus (opposite of forming a real image)
Draw a diagram of a converging lens
Draw a diagram of a diverging lens
What can we draw ray diagrams to find?
The image that different lenses produce with objects in different positions and the nature of the image formed
What is the principal axis?
The straight line that passes along the normal at the centre of each lens surface
How many construction rays do ray diagrams use?
3 constructive rays from a single point on to the object
What three construction lines should you include in your ray diagram?
- A ray parallel to the principal axis that is refracted through the principal focus
- A ray through the centre of the lens that travels straight on, without refraction
- A ray through the principal focus that is refracted parallel to the principal axis
What type of lens does a camera use?
Converging
What type of image does a camera produce?
A real image
What does a camera produce the image onto?
A film or an array of CCDs
What is a magnifying glass?
A converging lens that is used to form a virtual image of an object
What type of lens is the eye lens?
A variable focus lens
Describe the journey a light ray takes through the human eye
- Light enters the eye through the cornea. The cornea and the eye lens focus the light onto the retina.
- The iris adjusts the size of the pupil to control the amount of light entering the eye.
- The ciliary muscles alter the thickness of the lens to control the fine focussing of the eye. They are attached to the suspensory ligaments.
What is a ‘near point’?
The nearest point to an eye at which an object can be seen in focus by the eye.
What is a ‘far point’?
The furthest point from an eye at which an object can be seen in focus by the eye
What is the near point of the human eye?
25cm
What is the far point of the human eye?
Infinity
What is range of vision?
The distance from the near point of an eye to the far point.
What is the range of vision of the human eye?
25cm to infitity
List all the parts of the human eye
- Cornea
- Iris
- Pupil
- Eye lens
- Ciliary muscles + suspensory ligaments
- Retina
- Blind spot
- Optic Nerve
What does each part of the human eye do?
- Cornea - transparent layer that protects eye and helps to focus light onto retina
- Iris - coloured ring of muscles that control amount of light entering eye
- Pupil - Central hole formed by iris. Light enters the eye here.
- Eye lens - Focuses light onto retina
- Ciliary muscles + suspensory ligaments - Attached to suspensory ligaments. The muscles change the thickness of the eye lens
- Retina - The light-sensitive cells around the inside of the eye
- Blind spot - Region where the retina is not sensitive to light (no light-sensitive cells present)
- Optic nerve - Carries nerve impulses from the retina to the brain
Draw and label a diagram of the human eye
What is the power of a lens?
The focal length of the lens in metres.
The unit of lens power is dioptres, D
What is short-sight?
An eye that cannot focus on distant objects but can focus on near objects
What causes short-sight?
The eyeball being too long or the eye lens being too powerful
What type of lens can be used to correct short sight?
A diverging lens
What is long site?
An eye that cannot focus on nearby objects, but can see objects further away clearly
Why are distant objects blurred for those with short sight?
Because the uncorrected image is formed in front of the retina (not on it)
Why are distant objects blurred for those with long sight?
Because the uncorrected umage forms behind the retine (not on it)
What causes long sight?
The eyeball being too short of the eye lens being too weak
What type of lens can be used to correct long sight?
A converging lens
What is the focal length of a lens determined by?
The refractive index of the material from which it is made
The curvature of the two surfaces of the lens
For a lens of a given focal length, the greater the refractive index of the lens material…
…the flatter and thinner the lens can be manufactured
