Light - The Electromagnetic Signal

Along the visible band (350 nm - 780 nm) (1 nanometer=$10^{-9}$ meter) of the electromagnetic spectrum, there are only about 128 fully saturated colors that can be distinguished by the human eye. It is the spectral composition, the energy spectral distribution $L(\lambda)$ of the light that is the signal the eye receives for the brain to process. Sometimes $L(\lambda)$ may be referred to as the color, but this is to be distinguished from the actual color we perceive from this light, because the energy spectral distribution is not the only factor that determines the perceived color, as we will discuss later.

Newton's experiments and contributions (1704):

• Decomposition of white light by a prism into many spectral lights, and reconstruction of a white light from the spectral lights by a second prism; A spectral light can not be further decomposed.
• A spinning round plate with sections painted in different colors would look white;
• Realization that perception of color is caused by the spectral composition of the light.