The frequency with which the electromagnetic wave oscillates is also used to characterize the radiation. The product of the frequency (ν) and the wavelength (λ) is equal to the speed of light ( c ); ., νλ = c . The frequency is often expressed as the number of oscillations per second, and the unit of frequency is hertz (Hz), where one hertz is one cycle per second. Since the electromagnetic spectrum spans many orders of magnitude, frequency units are usually accompanied by a Latin prefix to set the scale of the frequency range. ( See measurement system: The metric system of measurement: The International System of Units for a table of the prefixes commonly used to denote these scales.)