§10.6 Photochemistry 6.3 Photochemistry (1)The first law of photochemistry Light must be absorbed by a chemical substance in order to initiate a photochemical reaction Grotthuss and draper, 1818:6.3 Photochemistry (1) The first law of photochemistry: Grotthuss and Draper, 1818: Light must be absorbed by a chemical substance in order to initiate a photochemical reaction. §10. 6 Photochemistry