Bioluminescence The ability of some animals and plants to produce light. The light is generated by chemical reactions.

Concave lens A shaped piece of glass or plastic that is thicker at its edges than at its centre. It makes light rays diverge (spread out). When you look through a concave lens, objects appear smaller than they really are.

Convex lens A shaped piece of glass or plastic that is thicker at its centre than at its edges. It brings light rays together at a single point called a focus. When you look through a convex lens, objects appear larger than they really are.

Diffraction The spreading out of light rays as they pass through a narrow gap.

Focus The point where rays of light meet when directed by a lens or mirror.

Lens A shaped piece of glass or plastic that bends light rays.

Light A kind of energy that our eyes can detect, enabling us to see. It is mostly produced by very hot things. It travels in straight lines, but it can reflect (bounce) off objects, diffract (spread out) and refract (bend). In fact, we can only see an object when light reflects from it and into our eyes, or if it produces light itself. Sunlight is not colourless, but is made up of all the colours of the rainbow: the spectrum of light.

Light filter A piece of coloured, transparent plastic or glass that allows light of its own colour to pass through it, but stops all other colours. Light filters are often used in stage lighting, to change the colour of a light. If a blue filter is placed over a white light, the light that shines on to the stage will be blue.

Luminescence Light that is created at low temperatures, usually caused by certain chemical reactions.

Mirage A trick of the eye, that makes far away objects look much closer than they really are. Mirages happen when light from the sky is reflected off a layer of hot air just above the ground or the sea. They are commonly seen in deserts.

Mirror A sheet of glass with a thin layer of metal on the back. Mirrors form images by reflecting light from the surface of the metal.

Opaque A material that does not allow light to shine through it.

Optics The branch of science which deals with the behaviour and properties of light. It includes the construction of instruments, such as microscopes and telescopes, used to detect light.

Pigment A substance that is added to another substance to give it colour.

Primary colours Any of the three colours that can be mixed together to form all other colours. The primary colours of light are red, green and blue. They can be mixed to make any colour. When all three are mixed together they make white light. In paints, which contain pigments, the primary colours are red, yellow and blue.

Prism An angled block of transparent material such as clear glass or plastic. As light waves pass through a prism, they change speed and are bent, or refracted. Longer waves of red light refract least. Shorter waves of violet light refract most. The other colours of the spectrum spread out in between.

Rainbow An arch of colours (actually part of a complete ring, which is partly hidden by the horizon) that forms across the sky when the Sun shines from behind you at rain falling in front of you. The raindrops act as millions of tiny prisms, each splitting the white light in sunlight into every colour of the spectrum.

Reflection The bouncing of light rays off an object. Smooth, shiny, light surfaces reflect more light than rough, dark ones.

Refraction The bending of light rays as they travel through different transparent materials. This occurs because light travels at different speeds through different materials. At the boundary between two materials, the light changes speed and a light ray that strikes the boundary at an angle is bent from its straight path.

Shadow A dark shape or area produced when an opaque object stands between a surface and a source of light. Shadows are formed because light can only travel in straight lines and cannot bend around the opaque object.

Spectrum of light The section of the electromagnetic spectrum that we can see. It is formed of all the different colours that make up white light: red, orange, yellow, green, blue, indigo and violet.

Speed of light The speed at which light travels through empty space. This is about 300,000 km/s—about seven and a half times around the Earth in one second. The speed of light is the speed limit for the Universe: nothing can travel faster.

Translucent A material that allows light to pass through it, but which spreads the light rays out in all directions. We can make out shapes behind translucent materials but we cannot see them clearly. Tracing paper and some kinds of ice are both translucent materials.

Transparent A material that transmits light, or allows light to pass through it. We can easily see through transparent materials, such as glass. We can see the glass because a tiny amount of light is reflected off the material.

Consultant: Mike Goldsmith