Eye Defects

Why do people wear glasses?

Tuesday, 14 April 2015

By the end of todays lesson…..

To be able to name two common eye defects.

To be able to describe where the image is formed when a patient suffers from a named defect.

To be able to explain how the defect may be amended using a lens.

Correcting Eye Defects

Short Sight – Myopia

• Can see close up objects, but has difficulty in seeing distant objects.

Caused by: • The eyeball being elongated -

so that the distance between the lens and the retina is too great.

• The lens being too thick and curved - so that light is focused in front of the retina.

Corrected by: • A diverging (concave) lens

Long Sight – Hypermetropia • Can see distant objects, but not

things close up

Caused by: • The eyeball being too short - so the

distance between the lens and retina is too small.

• A loss of elasticity in the lens - meaning it cannot become fat enough to focus (which is often age-related).

Corrected by • The lens focuses light behind the

retina instead of onto it. Long-sightedness is corrected by putting a converging lens in front of the eye, as shown in the diagram.

Short Sight – Myopia

Long Sight – Hypermetropia

The eye can alter the shape and curvature of the lens to adjust the degree of refraction. This is called accommodation.

Key Ideas

• The near point is the closest an object can be from the eye without the object appearing blurred.

• The far point is the furthest an object can be from the eye without it appearing blurred.

• The distance between the near point and the far point is called the range of vision.

Comparing a camera to our eyes!

Camera Part Function in Camera Similar part in the eye

Lens To focus light onto the photosensitive surface at the back of the camera. This can either be photographic film or a CCD (charge-coupled device).

Focussing Dial Allows the user to adjust the focus for nearer or more distant objects.

Aperture Allows the user to adjust the amount of light entering the camera in different light conditions.

Photosensitive Surface Detects and records the light which is focused onto it. This can be on photographic film, but digital cameras use CCDs which convert light into electrical signals which can be stored.

Comparing a camera to our eyes!

Camera Part Function in Camera Similar part in the eye

Lens To focus light onto the photosensitive surface at the back of the camera. This can either be photographic film or a CCD (charge-coupled device).

Lens - which focuses light onto the retina.

Focussing Dial Allows the user to adjust the focus for nearer or more distant objects.

Ciliary muscles - which stretch or slacken the suspensory ligaments to adjust the shape of the lens

Aperture Allows the user to adjust the amount of light entering the camera in different light conditions.

Iris - which adjusts the amount of light entering the eye through the pupil.

Photosensitive Surface Detects and records the light which is focused onto it. This can be on photographic film, but digital cameras use CCDs which convert light into electrical signals which can be stored.

Retina - which detects light and converts it into electrical impulses which are sent to the brain.

Think about…

• Which type of vision (near or distant) requires the most refraction?