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contains frequencies that differ from that of the incident light and which will be characteristic of the
substance that causes the scattering. The accepted mechanism of the Raman effect, is that a molecule
absorbs the incident radiation and as a consequence, is raised to a higher level of energy. The molecule
then emits light at the Raman frequency and falls to a new level, usually somewhere between the initial
and final states. If the frequency of the incident light is (vi) and that of the scattered light (vs) then the
Raman frequency (Av) is given by.

The situation is depicted in Figure 2.13.

Figure 2.13.
Different Forms of Light Scattering
If the excitation energy is (hvo), and the molecule is raised from ground state to an excited level and
then falls back to the ground state, the frequency of the light emitted will be the same as that of the
incident light and this phenomenon is called Rayleigh scattering. If, however, the light excites the
molecule from ground level and then falls back to an energy level of (hv1) then the frequency of the
emitted light will be (v -v ), and
0
1