The first law of photochemistry, known as the Grotthuss-Draper law (for chemists Christian J.D.T. von Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place. No absorption, no effect.
For example, green grass is green because chlorophyll (an organometallic complex) absorbs other types of light but reflects green. If you try to grow plants under green light (460nm to 600nm) - they will reflect or transmit it all and not be able to photosynthesise. No photochemical reaction will have occurred.
The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for photochemical reaction. This is also known as the photoequivalence law and was derived by Albert Einstein at the time when the quantum (photon) theory of light was being developed. Only one excited molecule per photon absorbed.
Photochemists use techniques such as:
- Quantum yield - finding what proportion of molecules react when a known flux of light is used to irradiate them.
- Flash Photolysis - using a high intensity Xenon flash lamp to initiate a reaction which can then be studied be techniques auch as absorption spectroscopy.