A band of colors as seen in a rainbow, produced by separation of components of light by their different degrees of refraction…
The entire range of wavelengths of electromagnetic radiation
The second half of the stimulus-response relationship in psychology
The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye and the emitted light is in the visible region. Wikipedia
The scientific study of sight and the behavior of light, or the properties of transmission and deflection of other forms of radiation.
A discrete quantity of energy proportional in magnitude to the frequency of the radiation it represents.