Unit 3 - Medical Physics

Question 1

Give 4 properties of X Rays.

Answer 1

They are absorbed by bone and metal. They have a wavelength the diameter of an atom. They are transmitted through soft tissue/skin. They affect photographic film. They are transverse electromagnetic waves. They are ionising.

Question 2

Why are The number of x Rays a person has monitored over time?

Answer 2

X rays are ionising and can kill healthy cells/cause damage to DNA/damage cells/cause cancer.

Question 3

Give 2 examples of medical uses of X rays.

Answer 3

Imaging bones. Imaging dental problems. Imaging the body (CT scans). Killing cancer cells.

Question 4

Give 2 precautions taken by professionals involved in using X rays.

Answer 4

Leave the room. Warning light above the door of a room when x rays are used. Wearing a lead apron to absorb x rays. Standing behind a screen.

Question 5

What are the advantages of CT scans over X rays?

Answer 5

Clearer, more detail, 3D image. CT scans can take images of slices of body. The whole body can be imaged.

Question 6

Describe ultrasound.

Answer 6

A longitudinal wave/vibration above 20 000Hz/ beyond upper range of human hearing. Sound humans can’t hear.

Question 7

How is an image formed using ultrasound?

Answer 7

Ultrasound waves are partially reflected when they meet a boundary between two substances. The time taken for this reflection (echo) gives the distance of the boundary. Reflected waves are collected and processed into an image by a computer.

Question 8

What is ultrasound used for?

Answer 8

Imaging babies in the womb (pre-natal scanning). Destroying kidney stones. Treating muscle damage, tendons, ligament damage. removing plaque from teeth

Question 9

What is refraction?

Answer 9

The bending of light moving from one substance (medium) to another.

Question 10

What is the name of the point which a convex lens makes light rays meet?

Answer 10

The focal point.

Question 11

Why is an image formed from a lens virtual?

Answer 11

The real light rays do not meet to make the image, virtual light rays meet instead.

Question 12

Why are some people are short sighted?

Answer 12

Their eye ball is too long or the lens is unable to focus light rays onto the retina.

Question 13

Why are some people long sighted?

Answer 13

Their eye ball is too short or the lens can not focus light rays onto the retina.

Question 14

Give the near point and far point of our vision.

Answer 14

Near point = 20 cm. Far point = infinity

Question 15

What is the power of a lens with a focal distance of 60cm?

Answer 15

1/0.6 = 1.67D (notice 0.6m not 60cm!)

Question 16

The image is focused onto the retina in our eye. What is it focused on in a digital camera?

Answer 16

The CCD - charged coupled device

Question 17

Which TWO parts of the eye refract light before it arrives at the retina?

Answer 17

The lens and the cornea.

Question 18

Which part of the eye changes the shape of the lens to focus close by and far away objects?

Answer 18

The ciliary muscles.

Question 19

How can a lens be made thinner and flatter but still have the power needed to refract light correctly?

Answer 19

The lens must have a higher refractive index.

Question 20

Give Two factors affecting the power of a lens.

Answer 20

The refractive index of the lens and the thickness of the lens.

Question 21

What is an X Ray?

Answer 21

A very high frequency electromagnetic wave.A transverse wave, vibrations are perpendicular to the direction of energy transfer.

Question 22

Why is a charge coupled device CCD useful in medical imaging?

Answer 22

Images can be formed and stored electronically, so easily shared and manipulated.

Question 23

How are ultrasound waves produced?

Answer 23

Electronic systems produce ultrasound, a longitudinal wave with vibrations parallel to the transfer of energy.

Question 24

What is the distance from a lens to the principle focus or focal point called?

Answer 24

The focal length.

Question 25

What does the power of a lens tell you?

Answer 25

How much the lens bends light rays. Greater the power the more it bends light.

Question 26

Give one property of a lens that affects its power.

Answer 26

The thickness of the lens. The curvature of the lens (how much it bulges outwards or caves inwards. The refractive index of the lens.

Question 27

What type of lens is found in a magnifying glass?

Answer 27

A convex lens or converging lens.

Question 28

The nature of an image is describe in 3 ways. What are they?

Answer 28

Virtual or real. Inverted (upside down) or upright. Relative size compared to the object (smaller or larger)

Question 29

Why does the shape of the lens in the eye need to be changed?

Answer 29

To allow light rays to focus/meet on the retina from objects close by (thick lens) and far away (thinner lens).

Question 30

What is the equivalent part to our retina in a camera?

Answer 30

Digital camera = CCD, Traditional camera = photographic film.

Question 31

Give two factors affecting the focal length of a lens.

Answer 31

The curvature of the two surfaces of the lensThe refractive index of the material the lens is made of.

Question 32

Describe what happens to a light Ray in an endoscope.

Answer 32

Light is totally internally reflected because it reflects from the optical fibre at an angle larger than the critical angle.

Question 33

Give a medical use for a laser.

Answer 33

It is an energy source used for cutting skin/tissue burning cells and sealing blood vessels. Lasers are also used in eye surgery to correct sight.

Question 34

Describe why light totally internally reflects in an optical fibre.

Answer 34

The light hits the edge of the fibre at an angle bigger than the critical angle and reflects rather than refracts out of the fibre.