BESB: How does the eye work

How does the eye work


by Dr. Stephen Westland  (

{eye diagram}

Almost the whole of the interior of the spherically-shaped eyeball is lined with a layer of photosensitive cells known collectively as the retina and it is this structure that is the sense organ of vision. The eyeball, though no mean feat of engineering itself, is simply a structure to house the retina and to supply it with sharp images of the outside world. Light enters the eye through the cornea and the iris and then passes through the lens before striking the retina. The retina receives a small inverted image of the outside world that is focused jointly by the cornea and the lens. The lens changes shape to achieve focus but hardens with age so that we gradually lose our accommodation. The eye is able to partially adapt to different levels of illumination since the iris can change shape to provide a central hole with a diameter between 2mm (for bright light) and 8mm (for dim light).

The retina translates light into nerve signals and consists of three layers of nerve-cell bodies. Surprisingly the photosensitive cells, known as rods and cones, form the layer of cells at the back of the retina. Thus, light must pass though the other two layers of cells to stimulate the rods and cones. The reasons for this backward-design of the retina are not fully understood but one possibility is that the position of the light-sensitive cells at the back of the retina allows any stray unabsorbed light to be taken care of by cells immediately behind the retina that contain a black pigment known as melanin. The melanin-containing cells also help to chemically restore the light-sensitive visual pigment in the rods and cones after it has been bleached by light.

The middle layer of the retina contains three types of nerve cells: bipolar cells, horizontal cells, and amacrine cells. The connectivity of the rods and cones to these three sets of cells is complex but signals eventually pass to the front of the retina and to the third layer of cells known as retinal ganglion cells. The axons from retinal ganglion cells collect in a bundle and leave the eye to form the optic nerve. The backward-design of the retina means that the optic nerve must pass through the retina in order to leave the eye and this results in the so-called blind spot.

The rods and cones contain visual pigments. Visual pigments are much like any other pigments in that they absorb light with absorption sensitivities that are wavelength-dependent. The visual pigments have a special property, however, in that when a visual pigment absorbs a photon of light it changes molecular shape and at the same time releases energy. The pigment in this changed molecular form absorbs light less well than before and thus is often said to have been bleached. The release of energy by the pigment and the change in shape of the molecule together cause the cell to fire that is, to release an electrical signal by a mechanism that is still not completely understood.

Content Last Modified on 2/1/2007 4:01:31 PM