We can also talk about the frequency, f , of the wave, which is the number of cycles or crests passing an observer per second. This is the universal constant called the speed of light in vacuum . Stars radiate their energy primarily in this band. So does the Sun. This is called the visible region.
Longer l are redder and shorter l are bluer.
The diagram below shows the electromagnetic spectrum. Note that the intensity or flux density is a measure of the amount of energy crossing unit area in unit time. The law is a simple consequence of the law of conservation of energy. This explains why distant sources of light are dim.