Medical applications of physics

Question 1

What are x-rays?

Describe their frequency and wavelength.

Answer 1

X-rays are part of the electromagnetic spectrum.

They have a high frequency and a very short wavelength.

Question 2

Give three properties of x-rays.

Answer 2

1) They affect a photographic film in the same ways as light.
2) They are absorbed by metal and bone.
3) They are transmitted by healthy tissue.

Question 3

For what are x-rays used?

Answer 3

X-rays are used to form images of bones on photographic film to check for fracture or dental problems.

Question 4

What are charged coupled devices (CCDs) used for?

Answer 4

CCDs can be used to form electronic images of x-rays.

Question 5

How do CT scanners use x-rays?

What can be done to some body organs made of soft tissue so that they can be seen on an x-ray image.

Answer 5

CT scanners use x-rays to produce digital images of a cross-section through the body.
Some body organs made of soft tissue, such as the intestines, can be filled with a contrast medium that absorbs x-rays so that they can be seen on an x-ray image.

Question 6

Why must precautions be taken when using x-rays?

What should workers do to protect themselves?

Answer 6

X-rays cause ionisation and can damage living tissue when they pass through it, therefore precautions must be taken when using them.
Workers should wear film badges and when possible use lead screens to shield themselves from the x-rays.

Question 7

In what other way may x-rays be used?

Answer 7

X-rays may also be used for therapy, they can be used to treat cancerous tumours at or near the body’s surface.

Question 8

The human ear can detect sound waves between which frequencies?
Sound waves of a higher frequency than this are called what?

Answer 8

The human ear can detect sound waves with frequencies between 20Hz and 20,000 Hz.
Sound waves of a higher frequency than this are called ultrasound waves.

Question 9

What can be used to produce ultrasound waves?
What happens when a wave meets a boundary between two different materials?
How may the results be used?

Answer 9

Electronic systems can be used to produce ultrasound waves.
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.

Question 10

What equation can be used to calculate the distance travelled by an ultrasound pulse?

Answer 10

s = v x t
s is the distance travelled in metres, m
v is the speed of the ultrasound wave in metres per second, m/s
t is the time taken in seconds, s

Question 11

In the time taken between an ultrasound a transmitter sending out a pulse of ultrasound and it returning to a detector, how far has it travelled?

Answer 11

It has travelled from the transmitter to the boundary and back, i.e. twice the distance to the boundary.

Question 12

In what two ways can ultrasound be used in medicine?

Why is ultrasound safer than x-rays?

Answer 12

1) It can be used for scanning soft tissue such as the eye and unborn babies.
2) It may also be used in therapy, e.g. to shatter kidney stones into small pieces.
Ultrasound is non-ionising, so it is safer than x-rays.

Question 13

What is refraction?

Why does refraction take place?

Answer 13

Refraction is change of direction of a light ray as it passes from one transparent substance into another.
Refraction takes place because waves change speed when they cross a boundary. The change in speed of the wave causes a change in direction, unless the waves are travelling along a normal.

Question 14

What happens to a light ray when it crosses from air to glass?
What is the refractive index of a substance?

Answer 14

When a light ray crosses from air to glass it is refracted towards the normal.
The refractive index of a substance is a measure of how much the substance can refract a light ray.

Question 15

By what equation is the refractive index given?

Answer 15

n = sin i/sin r
n is the refractive index of the substance
sin i is the sine of the angle of incidence
sin r is the sine of the angle of refraction.

Question 16

How is a light ray refracted when it crosses from glass to air? What is also seen?
What will happen as the angle of incidence in the glass is gradually increased?
What is this angle of incidence called?

Answer 16

A light ray is refracted away from the normal when it crosses from glass to air. A partially refracted ray is also seen.
If the angle of incidence in the glass is gradually increased, the angle of refraction increases until the refracted ray emerges along the boundary.
This angle of incidence is called the critical angle, c.

Question 17

What happens if the angle of incidence is increased beyond the critical angle?
What happens when this occurs?

Answer 17

If the angle of incidence is increase beyond the critical angle, the light ray undergoes total internal reflection.
When total internal reflection occurs, the angle of incidence is equal to the angle of reflection.

Question 18

The critical angle is related to the refractive index by what equation?

Answer 18

n = 1/sin c
n is the refractive index
c is the critical angle

Question 19

What is an endoscope?
What does the endoscope contain?
How can visible light be sent through the endoscope?

Answer 19

An endoscope is a device used to look inside a patient’s body without cutting it open or when performing keyhole surgery.
The endoscope contains bundles of optical fibres, these are very thin flexible glass fibres. Visible light can be sent along the fibres by total internal reflection.

Question 20

What may be used as an energy source in an endoscope?
What may this be used for?
What is done to produce maximum absorption?

Answer 20

Laser light may be used as an energy source in an endoscope to carry out some surgical procedures such as cutting, cauterising and burning.
The colour of the laser light is matched to the type of tissue to produce maximum absorption.

Question 21

How can eye surgery be carried out on the retina?

Answer 21

Eye surgery 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.

Question 22

What is the principal axis?

Answer 22

The principal axis is the line through the centre of the lens and at right angles to it.

Question 23

What three construction rays from a single point on the object are used to locate the corresponding point on the image in ray diagrams?

Answer 23

1) A ray parallel to the principal axis is refracted through the principal focus.
2) A ray through the centre of the lens travels straight on without refraction.
3) A ray through the principal focus is refracted parallel to the principal axis.

Question 24

What type of lens does a camera use?

For what does the camera use the lens?

Answer 24

A camera uses a converging lens to form a real image of an object on a film or an array of CCDs.

Question 25

How does light enter the eye?

What is the role of the cornea and the eye lens?

Answer 25

Light enters the eye through the cornea. The cornea and the eye lens focus the light on the retina.

Question 26

What is the role of the iris?

What is the role of the ciliary muscles and how are they attached to the lens?

Answer 26

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 focusing of the eye. They are attached to the lens by the suspensory ligaments.

Question 27

What does the normal human eye have a near point of?
What is the far point of the normal human eye?
What is the range of vision of the human eye?

Answer 27

The normal human eye has a near point of 25 cm and a far point of infinity, so its range of vision is from 25cm to infinity.

Question 28

What equation is used to calculate the power of a lens?

Answer 28

P = 1/f
P is the power of the lens in dioptres, D
f is the focal length of the lens in metres, m

Question 29

How are parallel rays that pass through a converging lens refracted?
What is this point called?
What is the distance from the centre of the lens to this point called?

Answer 29

Parallel rays of light that pass through a converging lens are refracted so that they converge to a point called the principal focus.
The distance from the centre of the lens to the principal focus is the focal length.

Question 30

Why is there a principal focus on either side of the lens?

Answer 30

As light can pass through the lens in either direction.

Question 31

For converging lenses:
If the object is further away from the lens than the principal focus, what type of image will be formed?
On what does the size of the image depend?

Answer 31

If the object is further away from the lens than the principal focus an inverted, real image is formed.
The size of the image depends on the position of the object. The nearer the object is to the lens the larger the image.

Question 32

For converging lenses:

If the object is nearer to the lens than the principal focus what type of image will be formed?

Answer 32

If the object is nearer to the lens than the principal focus, an upright, virtual image is formed behind the object. The image is magnified, the lens acts as a magnifying glass.

Question 33

How can magnification be calculated?

Answer 33

Magnification = Image height/object height.

Question 34

How are parallel rays that pass through a diverging lens refracted?
What is this point called?

Answer 34

They are refracted so that they diverge away from a point.

This point is called the principal focus.

Question 35

For diverging lenses:
Why is there a principal focus on either side of the lens?
How can the image produced by a diverging lens be described?

Answer 35

There is a principal focus because light can pass through the lens from either direction.
The image produced by a diverging lens is always virtual.

Question 36

How does short sight affect vision?
What is short sight caused by?
How can short sight be corrected?

Answer 36

A person with short sight can see close objects clearly, but distant objects are blurred because the uncorrected image is formed in front of the retina.
Short sight is caused by the eyeball being too long or the eye lens being too powerful.
Short sight may be corrected using a diverging lens.

Question 37

How does long sight affect vision?
What is long sight caused by?
How can long sight be corrected?

Answer 37

A person with long sight can see distant objects clearly, but close objects are blurred because the uncorrected image is formed behind the retina.
Long sight is caused by the eye ball being too long or the eye lens being too weak.
Long sight may be corrected using a converging lens.

Question 38

Which two factors is the focal length of a lens determined by?

Answer 38

1) The refractive index of the material from which the lens is made.
2) The curvature of the two surfaces of the lens.

Question 39

For a lens of a given focal length what effect does the refractive index of the lens material have on the way the lens can be manufactured.

Answer 39

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.