P3.1 Medical applications of physics Flashcards Preview

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Flashcards in P3.1 Medical applications of physics Deck (91):
1

Function of the lens

Focuses light onto the retina. It changes shape to focus light from objects at varying distances.

2

Function of the cornea.

refracts light as it enters the eye

3

Function of the pupil.

controls the intensity of light entering the eye.

4

Function of ciliary muscles

Cause changes in the shape of the lens, which allows the light to be focuses at varying distances.

5

Function of Suspensory ligaments

Connect the ciliary muscles to the lens and hold the lens in place

6

Function of the iris

Coloured ring of muscle which control the size of the pupil

7

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

8

What is refraction?

the change of direction of light as it passes from one medium to another.

9

How does a lens form an image?

by refracting light

10

What is the focal length?

the distance from the lens to the principal focus is called the focal length.

11

What are the 2 main types of lens?

converging, diverging

12

What shape is a converging lens?

convex

13

What shape is a diverging lens?

concave

14

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

15

In a converging lens, what affects the type of image that is formed?

the distance the object is from the lens

16

Describe the image produced by a diverging lens

-upright

-virtual

-smaller than the object- no matter where the object it

17

How do you calculate magnification?

image height (divided by) object height

18

What do you find hard to do if you are long sighted?

see objects close up

19

What causes longsightedness?

eyeball being too short or the eye lens being able to focus on nearby objects

20

What causes shortsightedness?

eyeball being too long or the eye being unable to focus on distant objects

21

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

22

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

23

Why can't longsighted people focus on nearby objects?

as their near point is further away than normal

24

Why can't shot sighted people focus on distant objects?

their far point is closer than infinity

25

What is the near point?

The closest distance the eye can bring into sharp focus

25 cm

26

What is the far point?

The furthest distance that the eye can focus comfortably

- infinity

27

Describe the type of image taken by a camera

real image, image is smaller than the object, image is inverted

28

How is the image formed by our eyes similar to the image formed by a camera?

both a real inverted image

29

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

30

X rays are part of the ........

electromagnetic spectrum

31

What are some properties of xrays?

short wavelength- about the same diameter as an atom

High frequency

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

32

What are xrays absorbed by?

metal and bone

33

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

34

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

35

What 4 things can xrays be used to do in medicine?

used in CT scans

detect bone fractures

detect dental problems

kill cancer cells

36

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

37

What are CCDs, and what do they do?

Charge-coupled devices

allows xrays to be formed electronically

They detect xrays and produce electronic signals which are used to form high resolution images

38

What do CT scans stand for?

Computerised Tomography scans

39

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

40

What is the range for human hearing?

about 20Hz to 20,000 HZ

41

Why can humans not hear ultrasound waves?

because they are at a higher frequency than the upper limit of hearing for humans

42

What happens when ultrasound waves meet a boundary between two different media?

they are partially reflected

43

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

44

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)

t-time (s)

45

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

46

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

47

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

48

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

49

What is the refractive index of a medium?

the ratio of speed of light in a vacuum to speed of light in that medium

50

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

51

What does a converging lens cause parallel rays of light to do?

converge (move together) at the principal focus

52

What does converge mean?

move together

53

What does a diverging lens cause parallel rays of light to do?

diverge (spread out)

54

what does diverge mean?

spread out

55

What is the axis of a lens?

a line which passes through the middle of the lens

56

Where is the principal focus of a converging lens?

a point where rays hitting the lens parallel to the axis all meet

57

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

58

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

59

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

60

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

61

What kind of image is formed when you look in a mirror?

virtual (because the object appears to be behind the mirror)

upright

laterally inverted

62

Is the image virtual or real when looking through an object through a magnifying glass?

virtual

63

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

64

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

65

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

66

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

67

What type of lens do magnifying glasses use?

a converging lens

68

How do you work out magnification?

magnification = image height (divided by) object height

69

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

70

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

71

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

72

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

73

The more powerful the lens.....

the more strongly it converges rays of light, so the shorter the focal length

74

For a converging lens, the power is.....

positive

75

For a diverging lens, the power is....

negative

76

How can you make a lens more powerful?

make the lens with more strongly curved surfaces

77

For a given focal length, the greater the refractive index the................ This means that .............

flatter the lens

lens can be manufactured thinner

78

How can you work out the refractive index using the critical angle?

refractive index = 1 (divided by) sin c

sin c - the critical angle

79

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

80

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

81

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

82

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

83

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

84

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)

85

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

86

Why do X-rays cause ionisation?

because they are high frequency, high energy waves

87

The shape of the lens is changed by the................This allows the lens together with the ..................... to focus light onto the retina

ciliary muscle

cornea

88

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

89

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

90

What evidence would scientists have collected to come to the conclusion that x-rays can cause cancer?

x-ray records of people with cancer

91

Explain the advantage of a CT scan compared to a x-ray

a CT scan gives a 3D image

therefore the image can be observed from different direction