Text: When a phosphor or other luminescent substance emits light, it gives, in most cases, an emission according to a fundamental law known as Stokes' Law. This law states that the wavelength of the flourescent (emitted) light is always greater than the wave length of the exciting radiation. It was first observed in 1852 in the memoir "On the Change of Refrangibility of Light" by Sir G. G. Stokes. In terms of energy the relationship states e em < e ab While Stokes' Law holds true for the majority of cases it does not follow in certain instances. In some cases the wave length is the same for both the absorbed and emitted radiation. That is, the efficiency appears to be perfect or unity. This is known as resonance radiation. In the other case Stokes' Law does not hold where energy emitted is greater than the energy absorbed. This is known as Anti-Stokes Emission. In 1935 Prileshajewa showed that there is an energy difference as much as 1.1v between the exciting light and the flourescence of aniline vapor. This added energy is attributed to additions from the internal energy of the molecule.

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