Photochemistry Examples and Laws


Photochemistry is a field of chemistry that includes the study of the chemical reactions that are started or effected by the light absorption. When a molecule gets light, photochemical reaction start, which resulted in  the production of

  • excited states
  • radical intermediates
  • or other products that differ from the reactants.

Examples of Photochemistry

The examples of photochemistry are given below :

  • Photosynthesis
  • Photovoltaic or solar cell


Photosynthesis is a phenomenon in which plants and other organisms changes light energy into chemical energy, forming oxygen as a side product. Chlorophyll, a pigment located in the chloroplasts of plants,

  • absorbs light to start a series of photochemical reactions that
  • produce glucose and other organic compounds.

Photovoltaic cells

These cells are also called as solar cells, and usually

  • convert light energy into electrical energy.

When light is fall on a semiconductor material, such as silicon, electrons are shift into excited from ground state and can flow across the material to produce an electric current.

Laws of photo chemistry

The laws of photochemistry are a group of postulates that describe the behavior of molecules when they gets light and undergo photochemical reactions. Important laws of photochemistry are given below:

  1. Grotthuss-Draper Law:
  2. Stark-Einstein Law:
  3. Law of Reciprocity:
  4. Norrish Type I and Type II Photochemical Reactions:
  5. Quenching:

Grotthuss-Draper Law:

This law describes that only a fragment of the radiation spectrum that is absorbed by a molecule can start a photochemical reaction. In other words, a molecule will not undergo a photochemical reaction without the absorption of  light of the appropriate wavelength.

Stark-Einstein Law:

This law links the speed  of a photochemical reaction to the number of photons engaged by the reacting molecules. It describe that one photon absorbed by a molecule will form one molecule of the photochemically active species.

Law of Reciprocity:

This law describes that the rate of a photochemical reaction is proportional to

  • the intensity
  • and duration of the light source,
  • but independent of the intensity of the light per photon.

Norrish Type I and Type II Photochemical Reactions:

These are two types of photochemical reactions, distinguished by their mechanism.

Type I reactions

It involve the  production of free radicals.

Type II reactions

It involve the transfer of energy

  • from an excited state molecule to a ground state molecule,
  • resulting in the formation of a product.


This point out the loss of energy by an excited state molecule  because of collision with added molecule. This process can be either

  • non-radiative (no emission of photon)
  • or radiative (emission of photon).


Grasping the laws of photochemistry is essential for guessing and regulating the  result of photochemical reactions. These laws can be used to

  • plan new equipments ,
  • introduce new technologies,
  • and enhance our understanding of natural processes such as photosynthesis.