Flashcards in P3.1 Medical applications of physics Deck (91):
Function of the lens
Focuses light onto the retina. It changes shape to focus light from objects at varying distances.
Function of the cornea.
refracts light as it enters the eye
Function of the pupil.
controls the intensity of light entering the eye.
Function of ciliary muscles
Cause changes in the shape of the lens, which allows the light to be focuses at varying distances.
Function of Suspensory ligaments
Connect the ciliary muscles to the lens and hold the lens in place
Function of the iris
Coloured ring of muscle which control the size of the pupil
Function on the retina
Images are formed on the retina, which is covered in light sensitive cells. These cells detect light and send signals to the brain to be interpreted
What is refraction?
the change of direction of light as it passes from one medium to another.
How does a lens form an image?
by refracting light
What is the focal length?
the distance from the lens to the principal focus is called the focal length.
What are the 2 main types of lens?
What shape is a converging lens?
What shape is a diverging lens?
What is a real image?
where light from an object comes together to form an image on a 'screen'
- like the image formed on the eye's retina
In a converging lens, what affects the type of image that is formed?
the distance the object is from the lens
Describe the image produced by a diverging lens
-smaller than the object- no matter where the object it
How do you calculate magnification?
image height (divided by) object height
What do you find hard to do if you are long sighted?
see objects close up
What causes longsightedness?
eyeball being too short or the eye lens being able to focus on nearby objects
What causes shortsightedness?
eyeball being too long or the eye being unable to focus on distant objects
What type of lens would you give someone who was shortsighted?
a diverging lens.
it diverges light before it enters the eye, which means the lens can focus it on the retina
What type of lens would you give to someone who was longsighted?
A converging lens
the light is refracted and starts to converge before it enters the eye. The image can then be focused on the retina
Why can't longsighted people focus on nearby objects?
as their near point is further away than normal
Why can't shot sighted people focus on distant objects?
their far point is closer than infinity
What is the near point?
The closest distance the eye can bring into sharp focus
What is the far point?
The furthest distance that the eye can focus comfortably
Describe the type of image taken by a camera
real image, image is smaller than the object, image is inverted
How is the image formed by our eyes similar to the image formed by a camera?
both a real inverted image
What does the film in a camera (the CCD) do and what is it similar to in our eyes?
it detects light focused on it
this is similar to the retina
X rays are part of the ........
What are some properties of xrays?
short wavelength- about the same diameter as an atom
they are transmitted (pass through) soft tissue
they are absorbed by metal and bone
They affect a photographic film in the same way as light, so they can be used to take photographs
What are xrays absorbed by?
metal and bone
Describe how xrays are used in CT scans
CT scans use xrays to produce high resolution images of soft and hard tissue
The patient is put in a cylindrical scanner, and an xray beam is fired through the body from an xray tube and picked up by detectors on the opposite side
The xrays tube and detectors are rotated during the scan
A computer interprets the signals from the detectors to form an image of a two-dimensional slice through the body
multiple CT scans can be put together to make a three-dimensional image of inside the body
Describe an xray photograph
the brighter parts of the xray image are where fewer xrays get through (eg bone)
This is called a negative image
What 4 things can xrays be used to do in medicine?
used in CT scans
detect bone fractures
detect dental problems
kill cancer cells
Describe how xrays can be used to treat cancer
xrays cause ionisation and high doses can kill living cells
1) the xrays are focused on the tumour using a wide beam
2) This beam is rotated around the patient with the tumour at the centre
3)This minimises the exposure of normal cells to radiation, and so reduces the chances of damaging the rest of the body
What are CCDs, and what do they do?
allows xrays to be formed electronically
They detect xrays and produce electronic signals which are used to form high resolution images
What do CT scans stand for?
Computerised Tomography scans
What precautions are taken when xray machines and CT scanners are in use?
Radiographers wear lead aprons, stand behind a lead screen, or leave the room while scans are being done
lead is used to shield areas of the patients body that aren't being scanned, and the exposure time to the xrays is always kept to a minimum
What is the range for human hearing?
about 20Hz to 20,000 HZ
Why can humans not hear ultrasound waves?
because they are at a higher frequency than the upper limit of hearing for humans
What happens when ultrasound waves meet a boundary between two different media?
they are partially reflected
What can the time taken for reflections of ultrasound waves to reach a detector be used to determine?
how far away the boundary between the the two media is
In the equation
s= v x t
What do each of the letters stand for and what are the units?
s- distance (m)
v- speed (m/s)
Describe how ultrasound waves can be used in medicine
Used to break down kidney stones- an ultrasound beam concentrates high energy waves at the kidney stone and turns it into sand like particles, which can pass out of the body in the urine.- relatively painless, doesn't need surgery
Pre-natal scanning of a fetus- ultrasound waves can pass through the body, but whenever they reach a boundary between 2 different media (like fluid in the womb and skin of the fetus) some of the wave is reflected back and detected. Echoes are processes by a computer and produce a video image of the fetus
Compare xrays, ultrasound and CT scans in terms of how safe they are
Ultrasounds are non ionising-safe
Xrays are ionising + can cause cancer if you're exposed too high a dose- not safe to use on developing babies
CT scans use more x ray radiation than normal xrays- patient exposed to even more ionising radiatio- so not taken unless they are really needed
Compare xrays, ultrasound and CT scans in terms of image quality
ultrasound images are fuzzy- difficult to diagnose some conditions
Xray photographs produce clear images on bones are metal but not much else
CT scans produce detailed images + can be used to diagnose complicated illnesses due to high resolution images. High quality 3D image can be used in the planning of complicated surgery
How do you work out the refractive index?
refractive index = sin i (divided by) sin r
sin i - angle of incidence
sin r- angle of refraction
What is the refractive index of a medium?
the ratio of speed of light in a vacuum to speed of light in that medium
What is the nature of an image defined by?
its size relative to the object, whether it is upright or inverted relative to the object and whether it is real or virtual
What does a converging lens cause parallel rays of light to do?
converge (move together) at the principal focus
What does converge mean?
What does a diverging lens cause parallel rays of light to do?
diverge (spread out)
what does diverge mean?
What is the axis of a lens?
a line which passes through the middle of the lens
Where is the principal focus of a converging lens?
a point where rays hitting the lens parallel to the axis all meet
Where is the principal focus of a diverging lens?
The point where rays hitting the lens parallel to the axis appear to all come from
What are 2 rules for refraction in a converging lens?
1) An incident ray parallel to the axis refracts through the lens and passes through the principle focus on the other side
2) An incident ray passing through the centre of the lens carries on in the same direction
What are 2 rules for refraction in a diverging lens?
1) An incident ray parallel to the axis refracts through the lens, and travels in line with the principal focus (so it appears to have come from the principal focus)
2) An incident ray passing through the centre of the lens carries on in the same direction
What is a virtual image?
is when the rays are diverging , so the light from the object appears to be coming from a completely different place
you can't project a virtual image on a screen
What kind of image is formed when you look in a mirror?
virtual (because the object appears to be behind the mirror)
Is the image virtual or real when looking through an object through a magnifying glass?
Describe how you would draw a ray diagram for an image through a converging lens
draw a ray going from the top of an object to the lens parallel to the axis of the lens
This ray is refracted and passes through the principal focus
draw another ray from the top of the object going through the middle of the lens . This ray passes straight through the middle of the lens and does not bend
Mark where the 2 rays meet. That's the top of the image
Describe how the nature of the image produced by a converging lens for an object placed at different distances from the lens changes
An object at 2F will produce a real, inverted image the same size as the object and at 2F
An object between F and 2F will produce a real, inverted image bigger than the object and beyond 2F
An object nearer than F will make a virtual image the right way up, bigger than the object, on the same side as the lens
Describe how you would draw a ray diagram for an image through a diverging lens
Draw a ray going from the top of the object to the lens parallel to the axis of the lens
This ray is refracted so it appears to have come from the principle focus. Draw a ray from the principle focus. Make it a dotted line before it reaches the lens
Draw another ray from the top of the object going straight through the middle of the lens. this ray does not bend
Mark where the refracted rays meet. That's the top of the image
Describe how a magnifying glass works
by creating a magnified virtual image
-The object being magnified must be closer to the lens than the focal length (distance from the lens to the principal focus)
-since the image produced is a virtual image, the light rays don't actually come from the place where the image appears to be
What type of lens do magnifying glasses use?
a converging lens
How do you work out magnification?
magnification = image height (divided by) object height
Describe how a camera forms images
-light from the object you are taking a photo of travels to the camera and is reflected by the lens , forming an image on the film
-the image on the film is a real image because light rays actually meet there
-the image is smaller than the object , because the object's a lot further away than the focal length of the lens
-the image is inverted
In a digital camera, what is the equivalent of the retina in the eye?
CCDs - they detect all the light focused on them and record it
In the equation to find the power of the lens
P = 1 (divided by) f
what do the letters stand for and what are the units?
P- power in dioptres (D)
f- focal length
What is the focal length of a lens determined by?
-the refractive index of the material from which the lens is made
-the curvature of the two surfaces of the lens
The more powerful the lens.....
the more strongly it converges rays of light, so the shorter the focal length
For a converging lens, the power is.....
For a diverging lens, the power is....
How can you make a lens more powerful?
make the lens with more strongly curved surfaces
For a given focal length, the greater the refractive index the................ This means that .............
flatter the lens
lens can be manufactured thinner
How can you work out the refractive index using the critical angle?
refractive index = 1 (divided by) sin c
sin c - the critical angle
Describe how lasers are used to surgically correct eye problems
By using lasers
laser- a narrow, intense beam of light
they cauterise small blood vessels as they cut the tissue which reduces the amount of blood the patient loses and helps to protect against infection
eye surgery- a laser can be used to vaporise some of the cornea and make it change shape- which changes its focusing ability. This can increase or decrease the power of the cornea so that the eye can focus images properly on the retina
Describe how optical fibres work
using total internal reflection
they carry visible light over long distances
they work by bouncing waves off the sides of a thin inner core of glass or plastic. The wave enters one end of the fibre and is reflected repeatedly until it emerges at the other end
they work using total internal reflection because the angle of incidence is greater than the critical angle
What happens if the angle of incidence is less than the critical angle when light passes out of a block of glass?
most light passes out (is refracted) but a little bit is internally reflected
What happens if the angle of incidence is equal to the critical angle when light passes out of a block of glass?
The emerging ray comes out along the surface. There's quite a bit of internal reflection
What happens if the angle of incidence is greater than the critical angle when light passes out of a block of glass?
No light come out of the block. It's all internally reflected - total internal reflection
What does the value of the critical angle depend on?
-refractive index- a dense material with a high refractive index has a low critical angle (will totally internally reflect more light)
this means if a material has a smaller critical angle, more light is totally reflected (can cause the material to look sparkly)
What is an endoscope and how does it work?
it is a thin tube containing optical fibres - lets surgeons examine inside the body
consists of2 bundles of optical fibres- one to carry light to the area of interest and one to carry the image back so it can be viewed - uses total internal reflection
image can be seen by an eye piece or displayed as a moving image on a screen
advantage- surgeons can perform many operations by only cutting small holes in people- key hole surgery
Why do X-rays cause ionisation?
because they are high frequency, high energy waves
The shape of the lens is changed by the................This allows the lens together with the ..................... to focus light onto the retina
Give some similarities between an eye and a camera
both use a converging lens
image formed is real
image is inverted
image in eye formed on the retina, image of camera is formed on film (CCDs)
amount of light entering the eye and camera can be controlled
Why does a radiographer go behind a screen before switching on a xray machine?
x-rays are ionising - damage cells
any stray x-rays are absorbed by the screen
which reduces the radiation dose to the radiographer
What evidence would scientists have collected to come to the conclusion that x-rays can cause cancer?
x-ray records of people with cancer