A Curved Piece of Glass

Light Takes a Turn

Light travels more slowly in glass than in air. So a beam that enters glass at a slant bends a little closer to straight-on, and bends back as it leaves. This is refraction.

How much it turns follows an exact rule, Snell’s law: light that meets ordinary glass 45° away from straight-on continues only about 28° away.

Rays traced surface by surface with Snell’s law

Coming to a Point

A convex lens is thick in the middle and thin at the edges. The nearer its edge a ray strikes, the more steeply tilted the glass, and the more the ray turns — so parallel rays all turn towards one point, the focus.

The distance from the lens to the focus is the focal length. The more strongly the lens is curved, the shorter its focal length and the more sharply it bends light.

An Upside-Down Picture

Light from each point of an object passes through the lens and gathers again at one point on the other side, as long as the object is farther away than the focus. Together those points make an image — upside down.

Bring the object closer to the lens and the image moves farther away and grows. Cameras and projectors focus by moving the lens nearer to or farther from the sensor or screen.

The Lens in Your Eye

Your eye makes images the same way: the cornea and the lens together bring light to a focus on the retina at the back, where it forms a small upside-down picture, which your brain has learned to read the right way up.

The eye focuses with about 60 dioptres, like a single lens with a focal length of about 17 millimetres. To look at something close, the lens inside it thickens a little to shorten its focus.

The eye in cross-section, schematic

Short Sight and Glasses

If an eyeball grows a little too long, light from far away comes to a focus in front of the retina and has spread out again by the time it gets there, so distant things look blurry. This is short sight, or myopia. An eye just 1 millimetre too long needs glasses of about −3 dioptres.

Glasses for short sight are concave lenses, thinner in the middle than at the edges. They spread the light slightly so the eye brings it to a focus right on the retina. By 2050, about half the people in the world are expected to be short-sighted.