Only light which is absorbed by a system can bring about a photochemical change.
Photochemistry
Field: Photochemistry
Sequence of Expressions
For every mole of a substance that reacts, a mole of photons of light is absorbed.
The amount of reaction product depends only on the total energy absorbed, not on the rate at which it is absorbed (Reciprocity Law).
Law
Kasha's Rule
Photon emission (fluorescence or phosphorescence) occurs in appreciable yield only from the lowest excited state of a given multiplicity.
The quantum yield of luminescence is independent of the wavelength of the exciting radiation.