In this topic you will learn about how light passes through a lens.

Materials that completely block light from passing through are called opaque. Materials that let all light through, so that objects on the other side can be seen clearly, are called transparent. Transparent materials may or may not color the light. Materials that let only some of the light through so that objects on the other side appear blurry are called translucent.

Polarization allows light vibrations to pass through material in only one direction. Normally light waves travel in all directions. Some materials, like some sunglasses, polarize light so that only the light vibrating up and down can pass through them. This blocks glare and other kinds of light that vibrate sideways.

Light rays can be bent. The bending of light rays as they pass from one substance into another is called refraction. Light traveling from one substance into a denser substance slows down. If light strikes the new material head-on, its direction is unchanged. If the light strikes at any other angle, it is refracted into a new direction.

Lenses are pieces of transparent materials with curved surfaces that use the refraction of light to make images. A concave lens is a lens that curves inward (is thicker at the edges than at the middle) and spreads light rays apart. A convex lens is a lens that curves outward (is thicker at the middle than at the edges) and brings light rays together.

Your eyes each have a convex lens that casts an image on the retina at the back of the eye. Images that form on the retina are sent to the brain by the optic nerve. Sometimes the eye lens does not focus the image on the retina. This can cause blurry vision. A concave lens is used to correct a nearsighted eye. A nearsighted person can see things close up, but has trouble seeing things far away. A convex lens is used to correct a farsighted eye. A farsighted person can see things far away, but has trouble seeing things that are close up.

People with poor vision can sometimes get operations to improve their sight. People who are blind often use Braille, a code of raised patterns of dots, to read. Guide dogs are specially trained to assist visually impaired people.

Lenses can be used to focus light and to form various images. The cornea and lens of your eye act as lenses to focus light rays on the retina of your eye—and help you see things clearly.