Human Eye

A lens made of glass has a fixed curvature. Once it has been ground and polished, its surfaces stay exactly as they are. So the focal length and the power of a glass lens never change on their own. A magnifying glass or the glass in a pair of spectacles does the same job for years.

Our eye also has a convex lens, but this one is living tissue. Its curvature can be changed when we look at a distant object and when we look at a nearby one, and as the curvature changes, the power of the lens changes too. This single difference is what makes the eye far more flexible than any glass lens.

A camera, a microscope and a telescope are optical instruments made by people. The eye does the same kind of work, but nobody had to build it, which is why it is called a natural optical instrument. The inside of the eye is filled with two clear fluids. The aqueous humor fills the space at the front, between the cornea and the lens. The vitreous humor fills the large space behind the lens. Together they press outwards evenly and give the eyeball its spherical shape.

Every part of the eye has its own job. Some parts let light in, some decide how much light enters, some bend the light so that it meets at one place, and some turn the light into a message the brain can understand.

Light coming from an object reaches the retina by passing through these, in this order:

• air
• the cornea
• the aqueous humor
• the eye lens
• the vitreous humor
• the retina, where the image is formed

Each time light passes from one medium into the next it can bend, and this bending is refraction. The parts of the eye work together to bend the light so exactly that the rays meet right on the retina. The message made at the retina is then carried to the brain by the optic nerve.

Every ray that enters the eye meets the cornea first. As light passes from air into the cornea, it bends a great deal. Of all the stages on the way (air, cornea, aqueous humor, lens and vitreous humor), the cornea refracts the light more than any other part. So the cornea acts like the main lens of the eye. It is a thin, clear dome, a little like the curved glass on a wristwatch, and if it becomes cloudy or scratched, vision suffers.

Look at your eye in a mirror. The round coloured part, brown or black or some other shade, is the iris. The black dot right in its centre is the pupil. The pupil looks black because it is a hole, and light that goes in does not come back out. In bright light, the muscles in the iris relax and the pupil becomes smaller. In dim light, the muscles in the iris contract and the pupil becomes wider. In this way a controlled amount of light enters the eye, whether you are standing in strong sunshine or sitting in a dark room.

This is one reason why, when you walk from sunshine into a dark room, you see almost nothing at first and then slowly begin to see. The pupil needs a little time to widen.

The cornea has already bent the light a great deal, but that alone does not bring the rays together exactly on the retina. The rays refracted from the cornea are refracted further in the lens so that they converge onto the retina. Think of it as teamwork: the cornea gives the big bend, and the lens does the fine adjustment that finishes the job.

The ciliary muscles sit in a ring around the lens and hold it in place. As they relax and contract, the thickness of the lens changes, and with it the curvature of the lens and its focal length change too. This is how the eye makes sure the light rays converge onto the retina. A glass lens has no such muscles, which is exactly why its shape can never change.

The retina works like a screen at the back of the eye. Only when the light rays are focused onto the retina is an image of the object formed. The retina has two types of cells: rod cells and cone cells. Their names come from their shapes. Rod cells are long and thin like little sticks, and cone cells narrow to a point like tiny cones.

When you see a red rose as red, your cone cells are at work. At dusk it becomes hard to tell the colours of things apart, yet you can still tell light from dark. Judging brightness is the work of the rod cells.

Once the retina has turned light into electrical signals, those signals have to reach the brain. The optic nerve does this, like a cable joining the eye to the brain. The eye collects the light, but it is the brain that works out what we are looking at.

Because the eye lens is a convex lens, the image it forms on the retina is inverted. When you look at a tree, its image on your retina has the top of the tree at the bottom and the roots at the top. Yet we see the tree standing upright, because our brain inverts the image again once the signals arrive through the optic nerve.