P3 Medical applications of physics

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• Created by: Phoebe
• Created on: 08-04-13 13:29

1.1 X-rays

X-rays are used in hospitals:

• to make images and CT scans
• to destroy tumours at or near the body surface

X-rays cause ionisation and can damage living tissue when they pass through it.

X-rays are absorbed more by bones and teeth than by soft tissues.

CT scans distinguish between different types of soft tissue as well as between bone (or teeth) and soft tissue).

Charge-couple devices (CCDs) can be used to form electronic images of X-rays

High frequency = short wavelength

Lead absorbs X-rays

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1.2 Ultrasound

Ultrasound waves are sound waves of frequency above 20000 Hz.

Ultrasound waves are used in medicine for ultrasonic scanning and the destruction of kidney stones.

Ultrasound waves are partly reflected at a boundary between two different types of body tissue.

An ultrasound scan is non-ionising so it is safer than an X-ray.

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1.3 Refractive index

Refraction is the change of direction of light as it passes from one transparent substance into another.

Refractive index, n, is a measure of how much a substance can refract a light ray.

n = sin i / sin r

Remember, a ray of light travelling along a normal is NOT refracted.

Refraction takes place because waves change speed when they cross a boundary

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1.4 The endoscope

The critical angle is the angle of incidence of a light ray in a transparent substance that produces refraction along the boundary.

Refractive index = 1/sin c      (where c is the critical angle)

Total internal reflection occurs when the angle of incidence of a light ray in a transparent substance is greater than the critical angle.

Total internal reflection only occurs for a ray travelling from a more dense to a less dense material e.g. from glass to air.

An endoscope is used to see inside the body directly (fibreoptic)

Optical fibres are thin, flexible, glass fibres

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1.5 Lenses

Parallel rays of light that pass through a converging lens are refracted so that they converge to a point (the principal focus/focal point)

A converging (convex) lens focuses parallel rays to a point called the principal focus.

A diverging (concave) lens makes parallel rays spread out as if they came from a point called the principal focus.

A real image is formed by a converging lens if the object is further away than the principal focus.

A virtual image is formed by a diverging lens and, if the object is nearer to the lens than the principal focus, by a converging lens.

Magnification = image height/object height

The distance from the centre of the lens to the principal focus is the focal length

<---------------> = converging lens     >---------------< = diverging lens

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1.6 Using lenses

A ray diagram can be drawn to find the position and nature of an image formed by a lens.

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

When an object is placed between a converging lens and F, the image formed is virtual, upright, magnified and on the same side of the lens as the object.

A camera contains a converging lens that is used to form a real image of an object.

A magnifying glass is a converging lens that is used to form a virtual image of an object.

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1.7 The eye

Light is focused onto the retina by the cornea and the eye lens, which is a variable focus lens.

Light enters the eye through the cornea. The cornea and the eye lens focus the light onto the retina. 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 eye by suspensory ligaments.

The normal human eye has a range of vision from 25cm to infinity.

Lens power (dioptres) = 1/focal length (metres)

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Diagram of the eye

Eye lens: Focuses light onto the retina

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

Ciliary muscles: attached to the lens by suspensory ligaments. The mucles change the thickness of the eye lens

Optic nerve: carries nerve impulses from the retina to the brain

Blind spot: region where the retina is not sensitive to light (no light-sensitive cells present)

Retina: the light-sensitive cells around the inside of the eye

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1.8 More about the eye

The focal length of a lens is determined by the refractive index of the material from which the lens is made and the curvature of the two surfaces of the lens.

A short-sighted eye is an eye that can only see near objects clearly. We use a diverging lens to correct it.

A long-sighted eye is an eye that can only see distant objects clearly. We use a converging lens to correct it.

The higher the refractive index of the glass used to make a spectacle lens, the flatter and thinner the lens can be.

The camera has a lens of fixed shape, but variable position.

The eye has a lens of variable shape, but fixed position.

For a lens of a given focal length, the greater the negative index of the lens material, the flatter and thinner the lens can be manufactured.

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More about Ultrasound

The minimum frequency of an ultrasound wave is 20,000 Hertz.

When a wave meets a boundary between two different materials, part of the wave is reflected.

The time it takes to reach the detector can be used to calculate how far away the boundary is.

In the time between a transmitter sending out a pulse of ultrasound and it returning to a detector, it has travelled from the transmitter to the boundary and back.

Humans can hear from 20 - 20,000 Hz

Distanced travelled = speed of wave x time taken.

Kidney stones = ULTRA painful

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