Visible light is an example of electromagnetic radiation. This radiation may be pictured as a wave travelling through space. Although light always travels at a fixed speed, its wavelength – defined as the distance between two successive peaks or troughs – is not uniquely specified. Different types of light can have different wavelengths. These differences manifest themselves as different colours in the visible spectrum. For example, red light has a slightly longer wavelength than blue light. The light that we receive from the sun is a mixture of all the different colours.

Electromagnetic radiation with wavelengths significantly longer or shorter than those associated with visible light also exists. Two examples are gamma rays and radio waves. All types of electromagnetic radiation carry a certain amount of energy. A gamma ray has a lot of energy whereas a radio wave carries a relatively small amount of energy. In a sense, we can imagine the energy as localized around the peaks and troughs of the wave. Thus the energy of a given type of electromagnetic radiation is specified by its wavelength; a shorter wavelength corresponds to a higher energy and vice versa. This follows since a shorter wavelength means that more crests and troughs will arrive in a given interval of time, so more energy will be received.

Light has a very important property in that it changes its direction of motion as it travels between regions of different density.