The frequency (n) and wavelength (l) of a wave are related by the equation
n = c / l
where c is the velocity of propagation of wave. For electromagnetic radiation in a vacuum, c = 3*108 m/sec. Frequency can also be converted to units of energy using the relation E = hn, where h is the Plank's constant (h = 6.63*10-34 J*sec) However, expression of a radiation as frequency (Hz) gives result with very large numbers; therefore it is common, specially for electromagnetic wave in microwave to X-ray range, the frequency expressed as a wavenumber (cm-1). The wavenumber (n') is defined as the inverse of the wavelength in centimeters.
n' = 1 / l = n / c
The wave number is thus the number of waves per centimeter.
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Q: What is the conversion factor between wavenumber (cm-1) and joules per mole (J/mol)?