Bending light

In this chapter

Light travels through different substances at different speeds
As light slows down it may change direction and bend
The 'bending' of light is known as refraction
Lenses refract light so that objects appear bigger or smaller
Our eyes have a convex lens, and lenses are used in microscopes, telescopes and eyeglasses

Introduction

As we know, light travels very fast - at just under 300 000 kilometres per second - and, as a rule, in a straight line. We also know, however, that light travels at different speeds through different substances. Light travels more slowly through water than air, for example. As light travels through different substances, it slows down and changes direction or 'bends' slightly. This 'bending' of light is known as refraction. When light rays from the sun hit water at an angle, the rays slow down and appear to bend (refract) and change direction. This is why if you look at a straw in a glass of water it can appear to be bent out of place. If you look through the glass and the water at the straw, for instance, you are seeing the light bend as it passes through air, glass and water.

See Image 1

Refractive index

We can measure the different speeds at which light travels through various transparent (clear) substances, such as glass, water, or plastic. Light travels at just under 300 000 kilometres per second through air, at around 225 000 kilometres per second through water and at around 200 000 kilometres per second through glass. The ratio of the speed of light to this slower speed is called the refractive index of the medium.

Snell's Law

People have always been observing and experiencing refraction, but the discovery of what really happens in refracting light was made by the Dutch mathematician Willebrord Snell (1580-1626). Snell discovered that there was a relationship between the angle at which a light ray struck the surface of a transparent substance and the angle at which it travelled through that substance. This relationship became known as the Law of Refraction or 'Snell's Law'. This explains why when light travels through a convex or 'converging' lens, there is a certain point at which the image flips upside down.

See video 'Bending light'

Lenses

By studying how light bends (refraction), scientists have been able to make lenses for use in technological devices like telescopes, binoculars, eyeglasses and microscopes.

Lenses are shaped pieces of a transparent material such as glass or plastic. They are curved on one or both sides and can be used to refract (bend) light so that objects appear larger or smaller. There are two main types of lenses: convex lenses and concave lenses. They are similar to convex or concave mirrors, except that lenses let light through instead of reflecting it.

Convex lenses

Lenses which curve outwards and are wider in the middle than around the edges are called convex lenses. In general, convex lenses make things look bigger. A magnifying glass is made from a convex lens. If a magnifying glass is held near an object, the object looks bigger because the lens bends the rays of light inwards.

When light rays hit a convex lens they are refracted by different amounts. As the light rays leave the convex lens they come together at a point called the focus. We have convex lenses in our eyes that help us to see.

Concave lenses

Lenses which curve inwards are called concave lenses. These lenses are thinner in the middle than around the edges and make objects look smaller. When light rays pass through a concave lens they are bent and spread out. They do not come to a focus point. Things look smaller through concave lenses, but these lenses can help to make details clearer.

Concave and convex lenses are used together to make very clear images in things like telescopes and microscopes. Convex lenses enlarge things and concave lenses make details clearer.

A microscope is a tube with lenses at each end. At the bottom of the tube is the objective lens which is a combination of lenses that are designed to give a clear and magnified image of the object. At the other end of the tube are the eyepiece lenses which focus and enlarge the image again.

See Image 2