# Electromagnetic radiation/Relationships

From Wikibooks, open books for an open world

**Electromagnetic radiation: 3. Relationships**

By definition, wavelength x frequency = speed. Since the speed of light c = 2.997458 x 10^{8} m/s, we have

- λ ν = c

Also, by Planck's law,

- E = h ν

where h is Planck's constant 6.62607 x 10^{−34} Js.

It follows that a photon with an energy of 1eV has a frequency of 1 eV/h = 2.41799 x 10^{14} Hz or about 242 THz and a wavelength of c.h/1 eV = 1.23984 x 10^{−6} m or about 1,240 nm or 12,400Å. As will be seen below, that would put the photon in the infrared range. In practice, photon energies are never quoted for such long wavelengths because they are so low that the photon nature of the radiation is unimportant.