The Eye and Other Applications Using Light - P3 AQA

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The eye- strcuture of the eye

Retina- The layer at the back of the eye which contains two light- receptive cells which- in the presence of light- release an electrical impulse to the brain to form an image.

Lens- This is a disc which changes in length and curveature in order to refract light to the retina.

Cornea- This is the front part of the eye which refracts light as it enters the eye.

Pupil/iris- The pupil is a hollow in the middle of the iris which allows light into the eye. The iris is the coloured part of the eye which controls how mcuh light enters the pupil.

Ciliary muscle- These are the muscles connected to the lens and suspensory ligaments which control the size and curveature of the lens.

Suspensory ligaments- These control how the ciliary muscle stretches and slackens.

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The eye- correction of vision

Convex and concave lenses are used in order to correct vision:

- Long sight, caused by the eyebal being too short, or the eye lens being unable to focus. A convex/converging lens is able to correct this as it brings together light rays to make the eye think that they are coming from an object at a further distance. Therefore, light is focused on the retina and images closer up can be seen.

- Short sight, caused by the eyeball being too long, or the eye lens being unable to focus. A concave/diverging lens is able to correct this as it separates light rays to make the eye think that they're coming from an object that is closer so the rays focus on the retina and images further away can be seen.

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The eye- range of vision

The eye is able to focus on images that are between the near and far point.

The near point is approximately 25cm and the far point is infinity

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The eye - eyes and cameras

The lens in a camera focuses light on photographic film or the CCD as the lens in an eye focuses light onto the retina. A camera's CCD is the same as the eye's retina, as this is what causes the reaction- the impulses that create the image or the photo.

The focusing screw on a camera can be adjusted for near and far objects. This is what the ciliary muscles do in the eye

The arpeture in the camera controls what light enters the camera as the iris does to the pupil.

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The eye - power of a lens

The power of a lens is given by:

P= 1/f

Where P is the power in dioptres(D) and f id the focal length in metres

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The eye - focal length

The focal length is determines by: the refractive index of the material from which the lens is made; the curvature of the two surfaces of the lens.

HT- For a given focal length, the greater the refractive index, the flatter the lens. This means that the lens can be maufactured thinner.

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Other applications using light - total internal re

Total internal reflection and critical angle:

refractive index= 1/ sinC

Where c is the critical angle

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Other applications using light - optical fibres

Visible light can be sent along optical fibres. E.g.

Total internal reflection (http://www.bbc.co.uk/staticarchive/6479bfbbce38595157f4ff1cd0111190edacd806.gif)

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Other applications using light - lasers

The laser is used as an energy source fro cutting, cauterising and burning.

Knowledge of how a laser works doesn't need to be known but understand that it can be used in eye surgery.

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