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The eye

The eye is for us probably the most important of optical instruments. Figure 1 is a simplified diagram of the human eye.

The human eye is about 2.5 cm in diameter and the pressure of the fluid within it maintains its near-spherical shape. The eye as a whole behaves like a thick lens. We will consider here only the Physics of various parts of the eye. For a more detailed account of the biology you should consult a biology textbook.

Parts of the eye:

cornea - a curved transparent membrane at the front of the eye; most of the refraction takes place here
iris - the coloured part of the eye
pupil - this is simply a hole through which light passes; it acts like a diaphragm and its diameter can be changed from about 3 mm to about 8 mm depending on the light intensity
lens - this is flexible and it focuses the image on the retina; the ciliary muscles around it contract to view near objects so squashing it and thus shortening its focal length
retina - the light-sensitive surface on which the image is formed; it is composed of many millions of light-sensitive nerve endings, rods that are sensitive to detail and cones that detect colour

The liquid between the cornea and the lens is known as the aqueous humour and that in the main body of the eyeball is called the vitreous humour. The refractive indices of the aqueous humour and the vitreous humour are equal (1.337), and that of the lens is 1.437.

Figure 2 shows the passage of light through a 'normal' eye, the curvature of the lens being adjusted to focus on near or distant objects.

In dim light 5% of the light energy falling on the eye is reflected by the cornea, 35% goes through the cornea and is absorbed by the black pigment on the inner surface of the choroids, 50% is absorbed by the lens and humours, and the final 10% is absorbed by the rods.

Defects of vision

(i) Short sight or myopia
The eye can see near objects clearly but not distant ones. This is due to the eyeball being too long (even a very small elongation is enough to produce myopia) and/or the eye lens being too strong (Figure 3(a)).

In a myopic eye the image of a distant object is formed in front of the retina and this can be corrected by using a concave lens (Figure 3(b)).

(ii) Long sight or hypermetropia
The eye can see distant objects clearly but not close ones. The eyeball is too short or the lens is too weak. The image of a near object is formed behind the retina (Figure 4(a)) and this defect can be corrected by using a convex lens (Figure 4(b)).

The far point of the eye is the most distant point that the eye can see clearly and this should ideally be at infinity.

The near point is the closest point that the eye can see clearly without strain and usually this is 25 cm from the eye (sometimes this is called the least distance of distinct vision).

(iii) Astigmatism

The eye can also suffer from astigmatism. This is usually caused by an imperfectly shaped cornea which causes the light to be refracted by different amounts in different planes. Opticians can test for this by getting the person to view a shape like that shown in Figure 5. A perfect eye will see a clear image while one suffering from astigmatism will give blurring in some directions.

(iv) Presbyopia

As people get older they may suffer from different degrees of a condition called presbyopia. In fact presbyopia is a Latin word meaning "old eyes".

Most of the focusing of our eyes is done at the cornea but the crystalline lens inside the eye is responsible for fine focusing. This crystalline lens changes its shape to allow us to focus on objects at different distances. To do this is must be elastic but as we get older the lens loses some of its elasticity and so it becomes more and more difficult to focus. This is known as presbyopia and the symptoms commonly first appear in people aged around 45.

(v) Colour blindness
Some people, about 17% of males and rather less females suffer from different degrees of colour blindness - an inability to recognise colours properly. I have red-green colour blindness and I once taught with a man who could not detect colour at all and so his view of life was rather like watching a black and white TV.
It is now possible to correct for colour blindness by wearing a specially tinted contact lens over one eye.

Scotopic and photopic vision

Our eyes are designed to respond to the full spectrum of sunlight and not to selected areas.

The sensitivity of the cones in your eye is known as the photopic response and refers to colour vision and the perception of fine detail.

The sensitivity of the rods in your eye is known as the scotopic response and refers to vision under conditions of low level light intensity – so called ‘night vision’.

Natural daylight has a scotopic to photopic (S/P) ratio of about 2.5 while some fluorescent lamps will be about half this.

As you can see from the graph photopic sensitivities are at a maximum in green light at about 555 nm which is the wavelength at which the cones in our retina are most receptive. However scotopic sensitivities peak about 50 nm lower in the blue-green region of the spectrum at 507 nm, where the rods are most receptive.

The size of your pupil is affected by scotopic light reducing it to a smaller size than for a similar intensity of white light. This lets less light into the eye and because the rods in the retina respond more readily to scotopic light you can retain the same level of visual performance at this lower light intensity.

‘Full spectrum’ fluorescent lights can be really helpful to people working under artificial lighting all day.


© Keith Gibbs