Light is the form of energy that our eyes can detect, enabling us to see. Among other ways, it is produced by very hot things—stars like the Sun, fire and the tiny wires inside electric light bulbs. Certain animals also have light-producing organs. Light rays can travel only in straight lines, although they can be reflected, diffracted (spread out) or refracted (bent). If they strike an object which does not allow light to pass through (an opaque object), a shadow is cast on the unlit side. Sunlight is made up of all the colours of the rainbow: the spectrum of light. Light from the Sun is essential to most life-forms on Earth. Plants use sunlight to make their food, a process called photosynthesis. Thus all plant-eating animals, together with other animals that eat plant-eaters, also depend on sunlight.

The speed of light

When we switch on an electric light, it seems that the room is filled with light instantaneously. But light rays do take time to travel from their source. They travel extremely quickly: about 300,000 kilometres (186,000 miles, or seven-and-a-half times around the world) per second in outer space. The speed of light is, in fact, the speed limit for the Universe: nothing can travel faster. Light waves can travel through empty space as well as through air, water or glass—although they travel more slowly through these. Because stars are very far from Earth—at least thousands of billions of kilometres—it is easier to give their distances in light years: the amount of time it takes for light to travel to us from them.

Refraction

Light rays bend, or refract, when they pass through different transparent materials. This is because light travels at different speeds through different materials. At the boundary between two materials, for example, air and water, the light changes speed slightly and, if it strikes the boundary at an angle, is refracted from its straight path. You can see this effect when looking at the bottom of a swimming pool from the poolside. It looks much shallower than it really is.

Focusing light

A lens, a shaped piece of glass or plastic, can bend light, either spreading it out or bringing it closer together. A convex lens, one that is thicker in the middle than at the edge, brings light rays together at a single point called a focus. If you hold a convex lens so that the object you are looking at lies between the lens and the focus, the object will appear larger and closer to the lens than it really is.

A simple magnifying glass is a convex lens, and is useful for studying minute detail as, for example, on a postage stamp or a tiny insect or flower. The eye contains a natural convex lens which focuses an image on to the retina at the back of the eye.

A concave lens is the opposite of a convex lens: it is thicker around the edge than in the middle. This kind of lens diverges (spreads out) light rays. It is used in glasses to correct short-sightedness.

Reflection

Although light rays can only travel in straight lines, they can be reflected. In fact, for us to see any object at all, some light must be reflected from it otherwise it would appear black. A smooth, white surface reflects more light than a rough, dark one, but a flat, shiny surface reflects light best of all. Mirrors, sheets of glass with a thin layer of metal on the back, are excellent reflectors.

You can use mirrors to see round corners or above or below where you are. A simple periscope has two mirrors set at an angle opposite each other. Light coming in at one end can be reflected through to an observer at the other.

Mirrors can also be convex or concave. The largest optical telescopes use concave (dish-shaped) mirrors to collect light from distant stars. The mirror is curved in such a way that the light rays are concentrated on a single point. Shaving mirrors, designed to produce a magnified image of your face, are also concave.

An example of a convex mirror (shaped like the back of a spoon) is a driving mirror. The driver can see a wider view than with a flat mirror.

When a pattern of light rays hits a shiny, flat, smooth surface like a mirror, the light is reflected without the pattern being altered. This is why the reflection of a lamp in a mirror is clear and sharp. This is called specular reflection. If the surface is rough, like a wall, however, the reflected light is jumbled. No sharp image of the lamp can be seen. This is called diffuse reflection. Many surfaces, such as plaster, marble or paper, reflect light diffusely. In fact, being able to see any object is mainly because light reflects diffusely. To see an object that does not make its own light, that object must have light shining on it.

Diffuse reflection is caused either by surface roughness or because of the surface's crystalline nature: light is scattered by the many minute faces of the crystals it is made up from. Perfectly flat surfaces, such as polished marble or gloss painted objects, can reflect light both specularly and diffusely.

Virtual image

The first law of reflection, known as Snell's Law, states that the angle of incidence (the angle at which a light ray strikes the surface of the mirror) equals the angle of reflection (the angle at which the light leaves the mirror). The angles are measured from a line at right angles to the mirror, called the normal. The second law states that both the reflected and incident rays, along with the normal, all lie in the same plane.

When looking at a flat mirror your eyes see light that appears to come from behind it. But there is no light coming from behind the mirror: the reflected light travels to your eye as if it comes from an object that lies where the image is. It is not a real image, but what is called a virtual image.

Bioluminescence

Some animals and plants are able to produce light, a phenomenon called bioluminescence. It is generated by chemical reactions in living cells. Female fireflies or glow worms (really beetles) emit light when they are ready to mate. Below 1000 metres (3300 feet), the ocean waters are completely without light. Here, many fish and other deep-ocean life-forms have special light-producing organs.