# Physics 3 Revision

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• Created on: 14-05-13 19:02

## Refraction and Lenses

There are two types of lenses:

-Converging Convex

-Diverging Concave

The direction of a light ray changes when it passes across a boundary between two transparent substances. This is refraction.

As a light ray moves from air to perspex, the ray moves towards the normal. Then, as the ray moves from perspex to air, the ray moves away from the normal.

If the angle of incidence = 0 degrees, the angle of refraction = 0 degrees

The incident ray and emergent ray are in the same direction and are parallel.

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Remember:

T - Towards

A - Air

G - Glass

A - Away

G - Glass

A - Air

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## Refractive Index

Refractive Index:

Refractive Index = sin i / sin r

i = angle of incidence          r = angle of refraction

The refractive index is different for every material, for example:

- Air = 1.0

- Water = 1.33

- Diamond = 2.5

- Perspex = 1.5

In materials with a higher refractive index, the slower the speed of light.

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## The Human Eye

We see things when light reflects off everything, into our eyes.

A beam of light passes through the cornea, aqueous humor, the lens and the virteous humor to reach the retina.The beam of light can change direction (be refrected) in all of these places.

When light hits the retina, light is converted, where electrical pulses are sent to the brain.

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## The Human Eye - Keywords

Ciliary muscle - controls the shape of the lens

Iris - gives the eye its colour and controls the amount of light entering the eye

Lens - changes shape, in order to focus the light on the retina

Optic Nerve - carries the nerve impulses from the eye to the brain

Aqueous Humor - the fluid in the outer cabity, adding to the eye's strength

Vitreous Humor - the fluid inside the eye

Cornea - forms the outer protective membrane of the eye and bends the light towards the pupil.

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## Range of Vision and Lens Power

Range of Vision:

The eye can focus on objects between th 'near point', which is approximately 25cm, and the 'far point' which is infinity.

Lens Power:

The power of a lens is given by:

P = 1 / t

P = power in diaptres, D

t = focal length, m

Power of converging lenses is positive and power of diverging lenses is negative.

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## Camera vs. Eye

Camera:                                                                    Eye:

-Lens                                                                         -Lens

-Can change focus                                                      -Can change focus

-Film                                                                          -Retina

-Inverted image                                                           -Inverted image

-No iris                                                                       -Iris

-Shutter                                                                      -Eyelids

-No muscles or ligaments

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