# P3

GCSE Edexcel Physics P3 overview

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• Created by: Nicole
• Created on: 03-04-13 16:45

## Eyesight

Lights enters your eyes through the retina.

Light rays pass through the cornear and the lens before reaching the retina at the back of the eye.

The lens in the eye is flexible so that images can be focussed on things from different distances.

The retina is where the image is formed. The image formed is converted into electrical impulses by cells and is carried to the brain.

The amount of light entering the retina is controlled by the iris. It makes the pupil larger or smaller.

To form a sharp image on the retina, light rays must converge on the retina. This means the path of the rays has to be changed by the eye.

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## Refractive Index

Refractive Index(n)    =      Speed of light in air (Va)
Speed of light in substance(Vm)

Material            Speed of light

Air                    300,000,000
Water               225,000,000
Diamond          120,000,000 (The densest material)
Perspex            200,00,000

n =    Sin i  (angle of incidence)
Sin r (angle of refraction)

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## Critical Angle

When a wave enters a medium where it's speed is greater, as the ange of incidence increases, so deos the angle of refraction

At angles greater than the crital angle, the wave is completely refracted rather than refracted. This is known as total internal reflection.

For a converging lens, parrallel rays of light are refracted and meet at the focal point.

The distance from the lens to the focal point is the focal point.

For a diverging lens, the focal point is the point from where the rays seem to be coming from after passing through the lens

Thinner lens = Larger focal length                        A virtual image can be projected

Power of lens =            1
Focal length(m)

Parallel rays are rays coming from infinity which will never meet

A more gently curving lens has a longer focal length

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## X- rays

Ionising radiation turns atoms into ions by removing some of their outer electrons

Some types of Em radition are ionising. E.g X-rays

The ability of x-rays  to ionise materials depends on their energy.
The more energy the x-rays have, the more ionising they are.

The energy of x-rays is related to their frequency
The higher the frequency, the higher the energy.

I =  Q
T

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