9. Eye defects, their causes and correction using convex and concave lenses

The eye contains a convex lens structure that, ideally, focuses the incoming light rays to produce a sharp image on the retina at the back of the eyeball.

The light receptors in the retina then produce and transmit the electrical signals to the brain giving you vision.

It is the energy of the light photons that cause a change in light sensitive molecules that trigger the electrical signal to the brain.

However, the eye does not always sharp images, but it is possible to correct these defects using glass lenses.

See also The eye - structure, function and vision defects GCSE level biology revision notes - This page deals with all the GCSE level biology required on the eye and more notes on other methods of dealing with eye defects.

Cause of vision defects causing lack of focus i.e. causing blurred image on retina

A malfunction of the eye in some way to give blurred vision because the incoming light rays do not form a sharp image on the retina.

 

1. Causes of short-sightedness (the medical term for short sightedness is myopia)

There are two principal reasons why a person can suffer from short-sightedness (both indicated on the diagrams).

Short sighted people can't focus correctly on distant objects.

There are many 'biological' causes to make a person short-sighted, but here we are most concerned with optics of the situation and how to use concave lenses to correct the defect.

The eyeball can be too long so the image is formed in front of the retina.

Distant objects will seem blurred, though close objects may be in focus.

The same effect is caused if the lens is the wrong shape i.e. it is too powerful - too thick - with too short a principal focus, so your vision is blurred.

This can be corrected with a diverging concave lens (see the diagrams and explanation below).

 

2. Causes of long-sightedness (the medical term for long sightedness hyperopia)

Long sighted people can't focus correctly on near objects.

There are many 'biological' causes to make a person long-sighted, but here we are most concerned with optics of the situation and how to use convex lenses to correct the defect.

There are two principal reasons why a person can suffer from short-sightedness (both indicated on the diagrams).

The eyeball may be too short so the image is formed behind the retina.

Distant objects might be in focus but any relatively close objects will appear blurred.

The same effect is caused if the lens is the wrong shape - too weak - too thin - with too long a principal focus, so your vision is blurred.

This can be corrected with a converging convex lens (see the diagrams and explanation below).

 

Using lenses to correct for eye defects

1. Correcting for short-sightedness

Reminder: Short sighted people can't focus correctly on distant objects.

The eyeball is too long or the lens to thick (too strong) to produce a sharp image on the retina.

Prior to correction, the focussed image is formed in front of the retina.

In order to move the image back to give it a sharp focus on the retina you need to diverge the rays. This is done with a diverging concave lens in front of the eye.

The diverged rays are then brought to a focus further back by the eye lens onto the retina.

The diagrams (1) show the normal correct function of the eye, the effect of short-sightedness and the correction produced by the concave lens.

 

2. Correcting for longsightedness

Reminder: Long sighted people can't focus correctly on near objects.

The eyeball is too short or the lens to thin (too weak) to produce a sharp image on the retina.

Prior to correction, the image is formed behind the retina.

In order to form a sharp image on the retina, you to use a converging convex lens in front of the eye to bring the rays to a focus further forward.

The combined converging power of the glass lens plus the eye lens bring the rays to a focus on the retina on the inner surface of the eyeball.

The diagrams (2) show the normal correct function of the eye, the effect of long-sightedness and the correction produced by the convex lens.

See also The eye - structure, function and vision defects GCSE biology revision notes

This page deals with all the GCSE biology required on the eye and more notes on other methods of dealing with eye defects.