Laws of Photochemistry

(1) Grotthuss-Draper law (First Law of Photo-chemistry):

Only the light which is absorbed by a molecule can be effective in producing photochemical changes in the molecule.
"When light y on any substance, only the fraction of incident light which is absorbed by the substance can bring about a chemical change, reflected and transmitted light do not produce any such effect."
This law is purely qualitative. It doesn't give any relationship between the amount of light absorbed by a system and the number of molecules reacted.

(2) Stark-Einstein's Law (Second Law of Photo-chemistry):

"It states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction."
^{Einstein applied quantum theory to photochemical reactions and gave the law.}^{" When an atom or molecule absorbs light of a given frequency, it absorbs one quantum only"}

The energy absorbed by one mole of the reacting molecules is

E = Nhv

Where N= number of mole

h= plank's constant

v= frequency

This energy is called one Einstein.

=11.97X10^-5/λ(m)KJ mol^-1