Medical Applications of Physiscs

X-rays are part of the electromagnetic spectrum. They have a high frequency and very short wavelength. Their propetrties include; they affect a photographic film in the same way as light, they are absorbed by metal and bone and they are transmitted by healthy tissue. They are used to check for fractures and dental problems.

Charged-couple devices can be used to form electronic images of x-rays. CT scanners are used in hospitals and use use x-rays to produce digital images of a cross section through the body. X-rays can also be used to destroy tumours at or near the body surface. So they can be used as treatment for cancer.

But x-rays can cause ionisation and can damage living tissue, when they pass through it. So people who work around x-rays need to take precautions. They should wear film badges and when possible use lead screens to shield them.

Sound waves that have a frequency higher than 20 000 herts are called ultrasound waves. When a wave meets a boundary between two different materials, part of the wave is reflected. The wave then travels back through the material to a detector. The time taken for it to reach the detector, can be used to calculate how far away the boundary is. An ultrasound scan is non-ionising, so it is safer than x-rays.

S = v x t

S = distance travelled (m)

V = the speed of the ultrasound wave (m/s)

T = time taken (s)

Ultrasound scans can be used for scanning unborn babies and soft tissue like the eye.

Refraction is the change in direction of light as it passes from one transparent substance to another. It takes because waves change speed when they cross a boundary. The change in speed causes a change in direction, unless the wave is travelling along a normal.a light ray will refract when it crosses from air to glass, it is refracted towards the normal.

The refractive index of a substance is a measure of how much it can refract light.

N = sin i

Sin r

N = refractive index

Sin i = sine x angle of incidence

Sin r = sine x angle of refraction

A ray of light travelling along a normal Is not refracted.

A light ray will refract when it crosses from air to glass, away from the normal. A partially reflected ray is also seen.
The critical angle is related to the refractive index:

N = 1 N = refractive index
Sin cv. C = critical angle

An endoscope is a device used to look inside a patient's body, without cutting it open. It contains optical fibres. These are very thin, flexible glass fibres. Visual light can be sent along them by total internal reflection. Laser light may b used as a source of energy in an endoscope, to carry out things like cutting, cauterising or burning. Eye surgery on the retina can be carried out by using laser light that passes straight through the cornea, but is absorbed by the retina.

Converging lens: they focus on parallel rays to a point called the principal focus. The light can pass through the lens in either direction. This means that there is a principal focus on either side. The image formed is real.

Diverging lens: the lens make the parallel rays spread out as if they came from the principal focus. They are refracted so that they diverge away from a point. The image produced is virtual.

Magnification = image height
Object height

Parts of the eye
Iris = coloured ring of muscle that controls the amount of light entering the eye.
Eye lens = focuses light onto the retina.
Retina = the light sensitive cells around the inside of the eye.
Cornea = light enters through here, it is a transparent layer that protects the eye and helps to focus light onto the retina.
Pupil = the central hole formed by the iris. Light enters the eye through the pupil.
Optic nerve = carries nerve impulses from the retina to the brain.
Blind spot = region where the retina is not sensitive to light.
Ciliary muscles = attracted to the lens by suspensory ligaments. The muscles change the thickness of the eye lens.
To find the power of the lens:
P = 1 F = focal length in metres
F P = power of lens in dioptres

Short sight may be caused by the eyeball being too long or the lens being too powerful. It can be corrected by using a diverging lens.

Long sight may be cause by the eyeball being too short or the eye lens being too weak. It can be corrected by using a converging lens.

When comparing the structure of the eye and the camera, the camera has a lens of fixed shape but variable position. The eye lens has a variable shape but fixed position.

Focal length of a lens is determined by: the refractive index of the material and the curvature of the two surfaces of the lens.