The Eye and Vision — Page 77, Lesson 116

Let’s start with the big picture. The eye is the organ that receives electromagnetic waves within the visual spectrum from the external world and passes them to the brain for interpretation into an image. In other words, it captures light energy and turns it into a picture your brain can understand. The basic structure is similar to a simple camera: it has an aperture, a lens, and a light-sensitive screen called the retina. Now, to keep tracking a moving object, the eyes need to act in harmony with one another. That means the brain coordinates the muscles of both eyes so they move together. In a fatigued person, this coordination sometimes fails, and the result is that quite differing images are transmitted from each eye. That leads to double vision. Let’s look at the first major part light encounters: the cornea. Light enters the eye through the cornea, which is a clear window at the front of the eyeball. The cornea acts as a fixed focusing device. It is responsible for between 70% and 80% of the total focusing ability of the eye. That focusing is achieved by the shape of the cornea bending the incoming light rays. Next, the iris and pupil. The amount of light allowed to enter the eye is controlled by the iris, which is the coloured part of the eye. It acts as a diaphragm. It does this by controlling the size of the pupil, which is the clear centre of the iris. The size of the pupil can change rapidly to cater for changing light levels. If the eye observes a close object, the pupil becomes smaller. If the object is at a distance, the pupil becomes larger. The amount of light can be adjusted by a factor of 5:1. But that factor is not sufficient to cope with the different light levels experienced between full daylight and a dark night. So a further mechanism is required. In reduced light levels, a chemical change takes place in the light-sensitive cells on the retina, which enables them to react to much lower light levels.