Light and Colour
The various properties of diffracted and polarized light, although they are only observed under special circumstances, are nevertheless sufficient to enable us to make many deductions concerning natural light. It is in consequence of the varying periods of vibration for different light waves that their wave-lengths differ. The colour of light is determined by its wave-length. But when we speak of colour we always include the perception of light by our eyes. The connexion is a very intimate one, but we are not able simply to identify the colour which the eye perceives with colour as defined in physics. When a physicist speaks of colour he refers to those colours which are produced when sunlight passes through a prism, or is diffracted by a grating. The various colours which different bodies are able to absorb or reflect are a characteristic feature of these substances, and they are responsible for their body colours.