THE HUMAN EYE

1. Basic anatomy of the human eye

The eye is actually a very complex optical system. In order to understand the basics of image formation, one has to know a bit about the anatomy of the human eye, referring to the diagram above.

Essentially the eye is a roughly spherical sac-like structure. The following are of special interest:

2. Image formation by the eye

Referring to the simplified diagram on the right, light rays travel into the cornea, and are refracted by the cornea and the lens. In a normal eye, the image is formed on the retina, and is upside down. Signals are transmitted from the retina to the brain along the optic nerve, where the data processing takes place (the image is perceived the right way up, for instance!)


3. Refraction defects

The normal eye can focus on a wide range of objects due to the lens being able to change its shape. Small CILIARY MUSCLES, when contracted, cause the lens to assume a more spherical shape. The eye is the able to focus down to clearly see an object at the normal NEAR POINT, which is about 25 cm. When the ciliary muscles are relaxed, the curvature of the lens increases, and the eye can focus on the FAR POINT, which in the normal eye, is on distant objects. The ability of the lens to change its focal length is called ACCOMMODATION.

     
     

The eye is subject to a number of refraction defects. In the picture above (top row, left), the image is formed in front of the retina, leading to a condition known as MYOPIA (commonly known as SHORT-SIGHTEDNESS. The diagram in the top row, right, the image is formed behind the eye. This known as HYPEROPIA (or FAR-SIGHTEDNESS). These conditions are typically due to the eye being slighly shorter (hyperopia) or longer (myopia) than normal.

These conditions are corrected by the wearing of spectacles. In the case of myopia, one uses diverging (concave) lenses, which cause the image to be formed FARTHER from the lens, right on the retina (bottom row above, left). Hyperopia is corrected by using converging (convex) lenses, which cause the image to be formed CLOSER to the lens, again, right on the retina (bottom row, right).

4. Additional questions