How Do Optical Glasses Work?

Optical glasses, usually called eyeglasses, are glass or plastic lenses held before the eyes in a frame. Eyeglasses are most commonly used to correct vision problems or to protect from sunlight.

Lenses

• In normal eyes, light enters through the cornea at the front and falls on the retina at the back of the eye, where an image is formed. Some people cannot see clearly because the shape of their eye puts the focal point of the light at the wrong place. Eyeglass lenses bend the light passing through them and change the focal point of the light, moving it to the correct place on the retina.

Correction

• In nearsighted eyes, the focal point of light falls in front of the retina. A concave lens moves the focal point backwards to the retina. For farsighted eyes, the focal point must be moved forward by a convex lens. Eyes with astigmatism have irregular surfaces in the cornea that distort the light as it enters. Specially shaped lenses offset the distortion and center the light correctly on the retina.

Protection

• Sunglasses protect the eyes against too much exposure to sunlight.

  Sunglasses use dark-colored lenses to reduce the amount of sunlight that reaches the eyes. Excessive exposure to sunlight is linked to several health problems, most notably cataracts, a clouding of the cornea. According to the National Eye Institute, 22.3 million Americans suffer from cataracts. To be effective, sunglasses must block both UV-A and UV-B rays.