AQA Physics Paper 3 - Medical Physics

X-rays are used in hospitals to...

image bones and destroy tumours near the body's surface.

X-rays are dangerous because...

they can ionise living tissue when they pass through it.

Bones appear white on the radiograph because...

x-rays are absorbed more by bones and teeth than by soft tissue.

CT scans provide better images than radiographs because

they can distinguish between soft tissue and provide a 3D image

CT scans are used less than ordinary radiographs

because the radiation dose received by the patient is far higher and the cost is more expensive.

Sound waves above the human hearing limit are referred to as

Ultrasound

The human hearing limit is

20-20,000Hz

Ultrasound waves are used in medicine for

prenatal scans and destroying kidney stones

Ultrasounds are safe for prenatal scans because

they are non-ionising

The image in an ultrasound is made by

the ultrasound waves being partially reflected at the boundaries between different tissue types.

to calculate the refractive index of a substance

you divide sin(i) by sin (r)

the refractive index

is a measure of how much a substance can refract light.

when you measure the angles i and r you must measure

the angle between the ray and the normal.

the critical angle

is the angle of incidence that causes a refracted ray to travel along the boundary

to calculate sin (c) you must

divide 1 by the refractive index of the substance

TIR stands for

Total Internal Reflection

For TIR to occur

The incidence ray must be in the substance with the higher refractive index and strike the boundary at an angle greater than the critical angle.

TIR is used to make this technology work

the endoscope

magnification =

image height / object height

a virtual image is formed

where the rays appear to come from

focal length

the distance from the centre of the lens to the principle focus (focal point)

real image

can be formed on a screen where the light rays meet

Iris

coloured ring of muscle that controls the amount of light entering the eye

Cornea

transparent layer that protects the eye and helps to focus light onto the retina

Lens

focuses light on the retina

Retina

the light sensitive cells at the back of the eye

Pupil

Light enters the eye here.

Suspensory ligaments

attach the ciliary muscles to the lens

Ciliary Muscles

Change the thickness of the lens

The unit of power for a lens

Dioptres (D)

a negative value of power is used for

a diverging lens

a positive value of power is used for

a converging lens

to calculate the power of a lens

1/ focal length

the normal near point of a human eye

25cm

the normal far point of a human eye

infinity

Short sightedness (Myopia)

When the uncorrected image forms in front of the retina

Long sightedness (Hypermetropia)

When the uncorrected image forms behind the retina

A high refractive index is good for spectacle makers because

it allows the glasses lens to be flatter and thinner