Focussen van licht in het oog

In a normal human eye, as light rays enter, they bend at the cornea, and the lens, to form an inverted image on the retina. The extent to which rays converge is called refractive power.

With the help of ciliary muscles, the lens can modify the refractive power of the eye to view near and far objects.

To focus on an object as close as 10 cm, the lens thickens, increasing the refractive power.

As the distance of the object from the eye increases, the lens flattens to reduce its refractive power. At around 6 m distance, the lens becomes the thinnest.

There are two main refractive problems of the eye.

In myopia—nearsightedness—the resting curvature of the lens is unusually thick, or the eyeball is abnormally long, with a focal point in front of the retina. This can be corrected using a concave lens.

In hyperopia—farsightedness—the lens is thinner than normal, or the eyeball is abnormally shallow. So the image converges behind the retina. This can be corrected using a convex lens.