State law of photochemisty
Answers
Answer:
Grothus-Draper Law
This law is referred to as the “first law of photochemistry”. This law was given by
Grothus in the year 1818 and was later reaffirmed by J.W. Draper in 1841. The law may
be stated as:
“When light falls on any substance, only a fraction of it is absorbed whereas the
rest is either reflected or transmitted. It is only the absorbed light which is effective in
bringing about a chemical reaction.”
It is not necessary that the absorbed light will always lead to a chemical reaction.
The absorbed light may simply get transformed into thermal energy or may bring about
phenomena such as fluorescence, phosphorescence, etc. It is also observed in few cases
that the reacting substance does not absorb the light energy directly. The energy is first
absorbed by some other substance which is present along with the reacting substance and
thereafter it is transferred into the reacting substance. This phenomenon is called
photosensitization.
(ii) Stark-Einstein Law
This is the “second law of photochemistry” and is also known as the “Law of
Photochemical Equivalence”. In the year 1913, Einstein after taking into consideration the
work of Stark enunciated the Law of Photochemical Equivalence which states that:
“Every atom or molecule which takes part in a chemical reaction absorbs one
quantum of the radiation which induces the reaction.”