Colour is the way our eyes see different wavelengths of light. Red light has the longest waves – about 700 nanometres, or nm (billionths of a metre). Violet light has the shortest waves – about 400 nm. Light that is a mixture of every colour, such as sunlight and the light from torches and ordinary lightbulbs, is called white light (see mixing colours).
Things are different colours because molecules in their surface reflect and absorb certain wavelengths of light.
Deep-blue printers’ inks and bright-red blood are vividly coloured because both have molecules shaped like fourpetalled flowers, with a metal atom at the centre.
Iridescence is the shimmering rainbow colours you see flashing every now and then on a peacock’s feathers, a fly’s wings, oil on the water’s surface or a CD.
Iridescence can be caused by the way a surface breaks the light into colours like a prism does (see spectrum).
Iridescence can also be caused by interference when an object has a thin, transparent surface layer. Light waves reflected from the top surface are slightly out of step with waves reflected from the inner surface, and they interfere.
The surface skin of water on some spilt oil interferes with the vibrations of light causing it to be split up into the colours of the spectrum.
The macaw gets its brilliant colors because pigment molecules in its feathers soak up certain wavelengths of light and reflect others, including reds, yellows and blues, very strongly.
Iridescence on a CD is a result of light waves reflecting from both the top surface and the inner surface. This causes the spectrum of light which is sometimes visible.
As a light source gets hotter, so its colour changes from red to yellow to white to blue.