Three typical waves of visible light are shown below. The nanometer , abbreviated as nm , is a unit of distance in the metric scale that is used to measure electromagnetic radiation. All visible light falls between the wavelengths of 400 and 700 nm. Rays at 400 nano meters are violet, then we go through the colors of the rainbow up to about 650 nano meters which is read, passing by green color in the middle. However, If you are looking at a red wall, this light consist of several different colors and hues of red, where the red is the dominant.
Natural light is never completely pure.
If we have a lot of light combined, at all nano meter wavelength, we get white light. Some artificial light sources, laser beams for example, are able to create completely pure light of a single wavelength. The human eye is a receptor organ for electromagnetic radiation. The human retina contains photosensitive cells, called rods and cones . These cells contain light-sensitive molecules referred to as photopigments in their outer segments.