Spectrum

Here we show the electromagnetic spectrum. Only a very small portion of it is visible light (wavelengths between 400 and 700 nm).

The different names given to the waves are also shown. Uses of different part of the spectrum:

Gamma rays: Medical treatment
X-rays: Medical imaging
Infrared: Heat lamps
Microwaves: Ovens, radar
Radio waves: TV, AM, FM

The relationship between frequency and wavelength is given by

\[ \rm \mathbf{c = \lambda f; \lambda = \frac{2 \pi}{k}; f = \frac{ \omega}{2 \pi}} \]

A plot of the absorption coefficient of water versus frequency shows why animal life adapted to the very narrow range of frequency (and wavelength) which we call the visible. As you can see in the plot, which is absorption versus frequency for water on a many decade log scale, there is an incredible deep hole right in the visible range. At higher and lower frequencies electromagnetic radiation is absorbed very strongly by water, which is of course the medium in which all animal life evolved.