Often times, it is useful to talk about frequency instead of wavelength. The frequency is the number of times per second that wave peaks pass through a fixed location. Using both the wavelength $ \lambda $ and the speed $ s$, the frequency $ f$ is calculated as:

$\displaystyle f = {s \over \lambda} .$ (4.1)

The speed of light in a vacuum is a universal constant $ c$ with value approximately equal to $ 3 \times 10^8$ m/s. In this case, $ s = c$ in (4.1). Light propagates roughly $ 0.03$ percent faster in a vacuum than in air, causing the difference to be neglected in most engineering calculations. Visible light in air has a frequency range of roughly 400 to 800 terahertz, which is obtained by applying (4.1). As light propagates through denser media, such as water or lenses, $ s$ is significantly smaller; that difference is the basis of optical systems, which are covered next.

Steven M LaValle 2016-12-31