There are two laws of photochemistry.
- The Grothus-Draper law and
- The Stark- Einstein law
The Grothus – Draper law
In the 19th century, two scientists enunciated this law. They found that all the light incident on a sample is ineffective in bringing the chemical change.
According to the Grothus-Draper law, only the light absorbed by a system can bring about a photochemical change. This law is also known as the Principle of Photochemical activation
Stark-Einstein law of Photochemical Equivalence
This law is also known as the Principle of quantum activation. In 1908 first enunciated by Stark, and then after 1912, Einstein gave the concept of energy quantum to photochemical reactions.
In the first step of the photochemical reaction, one molecule is activated by absorbing one quantum of radiation. The law does not imply that one molecule must react for each photon absorbed.
Assume that ν is the frequency of the radiation absorbed, and the corresponding quantum of energy absorbed per molecule is hν.
The energy E, absorbed per mole of the reacting substance, is given by,
Here, c = velocity of light
λ= wavelength
What is One Einstein?
The energy absorbed per mole of the reacting substance is called one Einstein, denoted as E.
The numerical value inversely to the wavelength of the light absorbed; the shorter the wavelength, the greater is the energy absorbed.
