Hue is perhaps the easiest characteristic of colour to identify and understand. It is basically the quality to which we refer when we talk about "colours" in everyday language. Hue is determined by the wavelength of the light. The human eye reacts differently to the various wavelengths in the visible spectrum, so when we see red or yellow we are actually detecting different wavelengths of electro-magnetic radiation. In the diagram, the top row contains four quite different or contrasting hues. Each one has a measurable and quite distinct wavelength and is therefore detected in a particular way by the human eye. The bottom row contains four slightly different versions of the same hue (red). Although the four lower rectangles might be described as containing different colours, they all have the same hue - red.