Home >> Optics, the eye

Eye biology

retina - a light sensitive region on the rear of the inner surface of the eyeball.

accommodation - the ability of the eye to produce a focussed image on the retina. This is done by altering the shape of the eye-lens by muscle in the shape of a ring, called the ciliary muscle. The ciliary ligament transfers force between the muscle and the lens.

iris - a ring of muscle controlling the amount of light entering the eye.

lens - made of clear cartilage. In old age the lens can become opague. In a simple procedure it can be replaced with a plastic lens.

cornea - front part of the eye. Most of the deviation of light coming from an object occurs at the air/cornea boundary. Old age/disease can cause the cornea to become fogged, eventually causing blindness. Transplants from cadavers can remedy this.

pupil - the space inside the iris. This appears black because it leads into the eyeball. Inside the eyeball is caverness and dimly lit from light entering.

humours - the aqueous and vitreous humours are clear liquids within the eye with similar refractive indices.

aqueous humour (1.33 )
vitreous humour (1.34 )
eye lens (1.41 )
cornea ( 1.38 )

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Visual Angle

The size of objects perceived by the eye depends on the size of their image on the retina.

Suppose an object subtends an angle θ at the eye, producing an image of height H_i on the retina.

If W_e is the width of the eyeball between lens and retina, then for small angles, to a good approximation:

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Angular Magnification (magnifying power)

This is used in optical instruments - microscopes, telescopes.

Angular magnification is the ratio of the angle subtended at the eye by the image (β^o), to the angle subtended by the object (α^o).

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Near Point

The near point (D) is the closest distance infront of the eye where a positioned object is in focus. It is sometimes referred to as 'the least distance of distinct vision'.

The near point varies from person to person, but a good average is 25 cm.

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Short Sight (Myopia)

As the name implies, a person with 'short sight' can see objects close up, but not in the distance.

Short sight is a result of an abnormally long eyeball or the eye lens being too strong. The consequence is that the image of a distant object is formed infront of the retina. The resulting image actually reaching the retina is therefore out of focus.

Correction of short sight is by concave lens. A concave lens is a diverging lens. So the effect is to push the image back towards the surface of the retina, where it will be perceived as in focus.

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Long Sight (Hypermetropia)

As the name implies, a person with 'long sight' can see objects far away , but not close up.

Long sight is a result of an abnormally short eyeball or the eye lens being too weak. The consequence is that the image of a distant object is formed behind the retina. The resulting image perceived on the retina is therefore out of focus.

Correction of long sight is by convex lens. A convex lens is a converging lens. So the effect is to pull the image back from behind the eyeball towards the surface of the retina, where it will be perceived as in focus.

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