Figure 7.1
Wave representation of electromagnetic radiation.Thus,
where I is the intensity and E the energy. Frequency and wavelength are simply related by the
expression
where c is the velocity of propagation of the wave in a vacuum (2.998 × 108 m s–^1 ) and is independent
of frequency. It is important to remember that energy and frequency are directly proportional but that
energy and wavelength are inversely related. A unit used to characterize radiation in some spectrometric
techniques is the wavenumber σ defined as the number of waves per centimetre and related to
frequency and wavelength by the expression
The various spectrometric techniques have developed independently, and at different times. They differ
as to whether frequency, wavelength or wavenumber is measured, with the result that comparisons
between techniques is sometimes difficult – equation (7.2) is a useful aid in this respect.
Quantum theory considers radiation as a stream of 'energy packets' – photons or quanta – travelling
through space at a constant velocity (c when in a vacuum). The energy of a photon is related to the
frequency of the radiation, as defined in wave theory, by the expression
where h is Planck's constant (6.6 × 10 –^34 J s).