What does the iris do?
Control the amount of light entering the eye
What form of radiation does an endoscope use?
What are the two types of ionising rays used to get rid of cancer?
Gamma and X-rays
What happens to the intensity of radiation as it gets further away from the source?
What happens to the strength of radiation as the density of the medium it passes through increases?
It gets weaker
How do you find the intensity of radiation?
Intensity = power of incident radiation / area
What is power measured in?
What form of radiation does a PET scanner use?
What type of radiation does a CAT scanner use?
What is intensity?
The strength of a wave
What is radiation?
Energy carried by waves or particles from a source
What is a diagnosis?
The identification of a medical problem by its signs and symptoms or a medical imaging scan
How do endoscopes produce an image?
Visible light reflects off features to form an image
What is the standard unit of area measured in?
Metres squared, m^2
What are some types of non-ionising radiation used in medicine?
Why do doctors place radioactive sources at different distances from a cancer tumour?
Because different cancer tumours are treated with different intensities of gamma radiation
What factors is intensity influenced by?
- distance from the source
- medium that it’s passing through
What is incident radiation?
The incoming beam of radiation
What happens to the intensity of radiation as it travels outwards?
What is intensity measured in?
Watts per metre squared, W/m^2
What part of the eye causes the size of the pupil to change?
What happens to the pupil when the iris gets smaller?
Why can lasers travel long distances?
They have a very high intensity
What does the pupil change size in response to?
What is the pupil?
A round hole in the centre of the iris of the eye
What does light enter your eye through?
What do light rays need to pass through to reach the retina?
The cornea and lens
What is the cornea?
The outer transparent layer of the eye
What does the cornea do?
Refract light entering the eye
What does the lens do?
Further converge light rays refracted by the cornea to focus them on the retina
Where is the retina found?
The back of the eye
What is the retina?
Tissue at the back of the eye that contains light receptors
What happens to an image once it reaches the retina?
It is converted into electrical impulses by cells in the retina
What nerve are electrical impulses formed at the retina carried by?
The optic nerve
What are ciliary muscles?
Muscles that relax or contract to change the shape of the lens of the eye
What happens to the pupil when the iris gets bigger?
What does it mean when a pupil constricts?
It gets smaller and lets in less light
Why do light rays have to converge before they reach the retina?
To form a sharp image
What happens if too much light enters the eye?
It can damage the retina
What does it mean when a pupil dilates?
It widens and lets in more light
What does it mean when light rays converge?
They are brought closer together
What process happens in the eye in order to converge light rays?
What two parts of the eye cause light rays to refract?
The cornea and the lens
Why do the ciliary muscles change the shape of the lens when the object being viewed is moved nearer or further away?
To keep the image in focus on the retina (depending on distance more or less refraction is needed)
What is the far point of an average adult human eye?
What is the near point of the average adult human eye?
What is the far point?
How far away you can focus on distant objects
What is the near point?
The closest you can see an un-blurred image
What happens to the lens when the ciliary muscles contract?
It gets shorter and fatter
What happens to the lens when the ciliary muscles relax?
It gets longer and thinner
What do the ciliary muscles need to do for near objects?
What do the ciliary muscles need to do for far objects?
Why would someone be short-sighted?
The eyeball is too long or the cornea is curved too sharply
Why would someone be long-sighted?
The eyeball is too short or the lens is not thick or curved enough
What does it mean when someone has short sight?
Objects a short distance away are clear but far-away objects are blurred
What does it mean when someone has long sight?
Distant objects are clear but near objects are blurred
Why can’t light rays from distant objects focus on the retina in short-sighted people?
The light rays refract too quickly in the eye so they are focused before they reach the retina
Why can’t light rays from near objects focus on the retina in long-sighted people?
The ciliary muscles are taught but the lens can’t bend the light enough so the rays are focused behind the retina
What type of lens is used to correct short-sightedness?
What type of lens is used to correct long-sightedness?
How does a diverging lens correct short-sightedness?
The diverging lens bends the incoming rays apart so that the cornea and lens can focus the rays correctly on the retina themselves
How does a converging lens correct long-sightedness?
It refracts the rays more to help the cornea and lens finish refracting the rays on the retina
What are three treatments for long and short-sightedness?
- contact lenses
- laser correction
What is another name for glasses?
What must all contact lenses do?
Allow oxygen to permeate the eye
Why is it important that contact lenses are cleaned regularly if they aren’t disposable?
To prevent infections
How does laser correction help correct vision?
It uses a finely controlled laser beam to reshape the cornea
What is a benefit of using a laser to perform eye surgery?
It can make precise incisions in tissue without damaging the surrounding area
How does changing the shape of the cornea during laser correction help improve vision?
It changes the point at which light rays converge inside the eye
What is the focal point?
The point at which nearly parallel light rays converge after passing through a lens
What is the focal length?
The distance from the lens to the focal point
What is the object distance?
The distance from the object to the lens
Where is the image from a converging lens formed?
The focal point
What happens to parallel rays of light that enter a converging lens?
They are refracted and meet at the focal point
How is the focal length of a diverging lens measured?
The point at which all the rays seem to have come from to the middle of the diverging lens
What is the power of a lens measured in?
What is another name for the focal point?
The principal focus
What will the faces of a powerful diverging lens look like in comparison to a weaker one?
They will be more sharply curved
What happens to the focal length of a diverging lens as its power increases?
It gets shorter
What happens to rays of light passing through a diverging lens when its faces are curved more sharply?
They diverge more
How is the power of a lens calculated?
1 ÷ focal length in metres
What power does a lens with a focal length 2 metres have?
What do you use to find out where an image will form when the light rays from that object aren’t parallel?
The lens equation
What is the lens equation?
1/f = 1/u + 1/v
What does f stand for in the lens equation?
What does u stand for in the lens equation?
What does v stand for in the lens equation?
Where are the object distance, image distance and focal length measured from?
What are all the distances in the lens equation measured in?
What is a real image?
An image that can be shown on a screen
What is a virtual image?
An image that can’t be shown on a screen
If the lens equation produces a positive image distance what type of lens was used?
Do real images have positive or negative image distances?
Do virtual images have positive or negative image distances?
Why does the lens equation produce a negative image distance when a diverging lens is used?
Because the image isn’t there (virtual image)
What does the law of reflection state?
Angle of incidence = angle of reflection
How can you predict the path of a reflected ray?
Using the law of reflection
What is reflection?
When a wave or particle bounces off a surface
Where are the angles of incidence and reflection measured from?
From the ray to the normal
What is the angle of incidence?
The angle between the normal and the particle or wave when it hits a surface
What is the angle of reflection?
The angle between the normal and the reflected wave as it leaves a surface
What is refraction?
The change of a wave’s direction of travel when entering a new medium, because a change in density creates a change in speed
What happens to a ray when it enters a denser medium?
It refracts towards the normal
Why does a ray refract towards the normal when it enters a denser medium?
Because it slows down
Why does a ray refract away from the normal when it enters a less dense medium?
Because it speeds up
What happens to a ray when it enters a less dense medium than the one it was in?
It refracts away from the normal
What is Snell’s law used for?
To describe the relationship between the angle of incidence and angle of refraction when a wave passes from one medium to another
What is another name for Snell’s law?
The law of refraction
What is Snell’s law?
sin(i) / sin(r) = constant
What is the constant in Snell’s law equal to?
The refractive index of each material
(nr ÷ ni)
(material ray is being refracted in ÷ material ray came from)
What does i in Snell’s law stand for?
Angle of incidence
What does r in Snell’s law stand for?
Angle of refraction
What is the refractive index of a material?
The ratio of the speed of light in a vacuum to the ratio of the speed of light in that material
What does the direction a ray takes when it reaches the boundary of a different medium depend on?
- the angle of the ray
- the speed at which the ray can travel in both media
What is the critical angle?
The smallest angle of incidence at which the angle of refraction is 90 degrees or total internal reflection occurs
What will happen when a ray enters a medium at less than the critical angle?
Part of the wave is refracted and part is reflected
What will happen when a ray enters a medium at an angle greater than the critical angle?
Total internal reflection
What will happen when a ray enters a medium at the critical angle?
It will refract at 90 degrees (travels along the boundary)
What is total internal reflection?
When all of a wave is reflected back from a boundary instead of being refracted, nothing passes through
What is the critical angle for most types of glass?
Why are diamonds cut to make them sparkle?
To increase the amount of total internal reflection
Why do diamonds sparkle so much?
Their critical angle is very low so a lot of light gets trapped and bounces around inside
How are diamonds cut so that they sparkle as much as possible?
At angles that increase total internal reflection
What does the size of the critical angle depend on?
How fast the rays can travel through the two media
What happens to the angle of incidence and refraction when a wave enters a medium where it speeds up?
What happens to the amount of refraction and reflection as the angle of incidence decreases?
There is more refraction and less reflection
What happens to the rays of the infrared source in a car rain sensor when it doesn’t rain?
Total internal reflection
How does the infrared detector in a car rain sensor see if it’s raining?
If it has been raining it detects less infrared from the source because it has been refracted
Why is the infrared ray in a car rain sensor refracted when it rains but totally reflected when it doesn’t?
The infrared source is positioned at an angle greater than the critical angle from glass to air so total internal reflection occurs. When it rains this angle is no longer greater than the critical angle (change to glass to water) so the ray refracts
What law can be applied to calculate a critical angle?
Snell’s law when sin(i) is replaced with sin(c)
What is the angle of refraction when a ray is travelling along the critical angle?
What is sin(90) equal to?
What is the refractive index of air?
What is the formula for finding a critical angle?
Sin(c)/sin(r) = nr/ni
When a refractive index increases, what happens to the critical angle?
It gets smaller
What is an optical fibre?
A glass fibre that transmits light from one end to the other by total internal reflection
What type of rays are used to transmit signals in optical fibres for telephones, TVs and other data communications?
Infrared and visible light
What are optical fibres in an endoscope used for?
- bringing light to the end of the endoscope
- bringing reflected light from the object back to the viewer so that they can see the image
What focuses the gathered light reflected off the inside of the body in an endoscope?
The eyepiece lens
What are optical fibres made of?
High transparency glass
In what form is information transferred down in bundles of optical fibres?
Pulses of light
Why are fibre optic cables faster than traditional copper cable telephone lines?
They can carry more signals
How is refractive index calculated?
What are the uses of lasers?
- laser eye surgery
- cutting through materials
What does it mean when a laser has low divergence?
It spreads out very little
What happens when an ultrasound wave enters a different medium than the one it was in?
It is partially reflected
Can ultrasound waves travel through solid objects?
What does the device in a medical ultrasound scanner do?
It both transmits and receives the ultrasound waves
How does an ultrasound scan produce an image of the inside of the body?
- the device emits ultrasound waves
- these go through the body and are reflected at the interfaces between different tissues
- the device detects the reflected waves
- these are converted to an image onscreen
What is a medical example of the use of ultrasound in both diagnosis and treatment?
The location of kidney stones and the use of high intensity ultrasound waves to break them up
What are the uses of the absorption of ultrasound energy in treatment?
- breaking up kidney stones
- treating injured muscles
- treating swollen tissues
How is ultrasound energy directed effectively?
- by focusing it
- by controlling its intensity