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Refraction | Light pathway through the eye

Refraction

Refraction is the way the eye bends light rays so they focus on the light sensitive cells in the back of the eye. There are three focusing factors in the eye:

• Shape of the cornea
• Power of the lens
• Length of the eye

If all three of these factors are accurate, the light rays will be focused on the retina properly and the person will see clearly. However, if the cornea is too steep or too flat or the eyeball is shorter or longer than normal, the person may have a focusing problem.

Light pathway through the eye

Light rays enter the eye through the cornea, which bends the light rays closer together so they can pass through the pupil. Muscles controlling the iris allow it to change the size of the pupil to control how much light enters the eye. When the pupil gets smaller, less light enters the eye, and when it gets larger, more light enters the eye. That's why the pupils get larger in a dark place - to allow more light in. When you are outside in bright sunlight or in a brightly lit place, the pupil gets smaller so less light enters. When you go from a dark place (such as a movie theater) to a bright place, you have to squint or close your eyes until your eyes have time to adjust to the change in light. When you first enter a theater, you may have had the experience of pausing for a few seconds until your eyes adjusted so you could find your seat. Your pupil is opening so more light is allowed into the eye.

After light goes through the pupil, it passes through a structure called the lens, which has two curved surfaces to bend the light rays more. The ciliary muscle is attached to the lens and changes its shape to bend light waves to focus on the retina. If you are looking at a nearby object, the lens thickens, but if you are looking at something farther away, the lens thins. The lens also inverts the image so the image that focuses on the back of the eye appears upside down.

Finally, light falls on the retina, the layer of the eye filled with light sensitive cells called rods and cones. The retina contains millions of these light-sensing nerve cells. The cones are concentrated in the macula and provide sharp central vision. They also detect colors and fine details. The rods are located outside the macula and extend all the way to the edge of the retina. They provide us with peripheral vision, allow the eyes to detect motion, and let us see in dim light and at night.

The rods and cones process light rays and convert them to electrical impulses that move along nerve fibers in the optic nerve to the occipital lobe of the brain. This lobe interprets the electrical impulses and gives us what we call vision.