Light - Lenses 1

Lenses are not new, the oldest lens known dates back almost 3000 years. The first written evidence from history of the use of lenses comes from Greek and Roman times. They were used by craftspeople to carry out fine work, for checking that official seals were authentic and occasionally to help with vision. They were also used as 'burning glasses' to concentrate the Sun's heat onto wood or other inflammable items to light fires. These early lenses were made from a naturally transparent material known as rock crystal which is a type of quartz.

Eventually, people learned how to make good quality glass and in the 11th and 12th centuries, monks cut glass spheres in half to form plano-convex (flat on one side, curved on the other) lenses to help them with producing their beautifully-illustrated illuminated manuscripts. Some of these were as good quality as lenses made nearly 800 years later in the middle of the 20th century. Glasses were invented in the 11th century but it wasn't until the end of the sixteenth century that people started to combine lenses to make optical instruments like the microscope and telescope.

Lenses have one or two curved surfaces and work because they refract (bend) light. Convex lenses bring parallel rays of light to a focus, known as the principal focus. The distance from the lens to this focal point is called the focal length of the lens. The focal length is much longer for thinner lenses - they bend light a lot less than thicker lenses. Convex lenses magnify but concave lenses make things look smaller. A convex lens can be used to produce an upside-down image on a screen; we say this is a real inverted image. It is impossible to produce an image on a screen using a concave lens so we say they produce a virtual image.

Light that hits the surface of the lens at right angles is not bent, however, if it hits at an angle, it bends towards the normal (an imaginary reference line at a right angle to the surface). The amount of bending is predictable and depends on two things: the angle at which the light ray hits the surface (angle of incidence); and the refractive index of the glass. The refractive index is measured by measuring several angles of incidence and the corresponding angle of refraction. Dividing sin i by sin r gives a number - the refractive index. This is simply a ratio and has no units.