What is Photochemistry?

Photochemistry is the part of chemistry that studies the various mechanisms of reactions that are triggered by the presence of light energy. It also helps to study the changes involved in a molecule that can undergo the absorption of radiation.

Energy is absorbed in the form of discrete quanta particles called photons. The equation that relates energy with frequency of the particles of the radiation:


Here, h = plank’s constant and v = frequency of the emitting particles.

When light is absorbed, electronic excitation takes place, wherein the electrons transit from the ground state energy level to the excited state energy level.

Laws of Photochemistry

Grotthus Draper Law

According to this law, only the radiation that is absorbed in the reacting molecule is responsible for the chemical change caused to that molecule as in some cases some amount of the radiation is reflected or used for some other process.

Stark-Einstein Law

According to this law, for each photon of light absorbed, only one molecule is activated for the reaction.

Photosensitized Reaction

A photosensitized reaction is a reaction that can take place by the presence of a sensitizer, which absorbs radiation and carries it to the reacting molecules.

A very common and easy example of photochemical reaction is photosynthesis. Plants use sunlight and water to carry out the process of converting easily available carbon dioxide to glucose. Another example is dissociation of hydrogen. Mercury vapor acts as a sensitizer that helps to dissociate the hydrogen molecule into its constituents. UV light itself can’t do it.

“The image that shows photochemical reaction”

Photochemical Reaction

Reactions that can take place by light radiation are called photochemical reactions. Usually the radiations involved in such reactions are ultraviolet radiation, visible light or infrared radiation.

For example, the decomposition of hydrogen iodide and hydrogen bromide need the presence of light to be carried out.

Photochemical Smog

Photochemical smog is a type of smog (smoke + fog) formed when nitrogen oxide present in the air reacts with the UV light emitted by the sun. It forms a brownish-gray haze that is prominently visible during the day especially in warm areas.

It has harmful effects on the ecosystem.

“The image that shows smog”

Quantum Yield

Quantum yield is a ratio of the number of reacting molecules on a time interval to the number of quanta absorbed in that same time.

According to Einstein law, each molecule absorbs only one photon. This is why quantum yield is unity, which in mathematical terms means the number one.

As all reactions are endothermic, meaning they absorb energy in order to be carried out, the quantum yield increases with respect to temperature. 

Photophysical Process

Electrons enter an excited energy state when they absorb the radiations present. During de-excitations, two phenomena can possibly take place. These are called photophysical processes.

  • Fluorescence: normal state is retrieved immediately.
  • Phosphorescence: It takes some time for the electron to go back to normal state.

Photochemical Process

This is a type of process which occurs when the electrons cause some chemical change before going back to their normal energy state from the excited energy state.

Organic Photochemistry

Many organic compounds undergo reactions that occur under the presence of ultraviolet light.

For example, the photochemical halogenation of alkanes takes place under the presence of light. It is the substitution reaction that occurs between alkanes and halogens. It cannot take place without light.

Photodissociation Reactions

A photodissociation reaction is a chemical reaction that involves the breaking down of a chemical compound under the presence of light. It takes place when photons interact with a molecule and results in breaking it into its respective constituents.

An example of a primary photochemical reaction is when propanone absorbs light and enters an excited energy state. It has enough energy to break a C-C bond to form a methyl radical and an ethanol radical.

“The image that shows photo-dissociation reaction”


Photobiology is the study of the interactions of light with living organisms. It deals with the harmful and good effects of light. There are both chemical and physical changes in biological systems due to the radiation absorbed. One of the very common examples is mutation. Mutation is when the gene alters itself in the DNA because of the absorbed radiation.


Photoisomerization is a form of isomerization that is caused by photoexcitation. There are both reversible and irreversible photoisomerization. The arrangement of the molecule changes but the molecular formula remains constant. The branch of science that studies the physical properties of light affected substance is called photo-physics.


Photocycloaddition is a reaction of cycloaddition. In this process, two unsaturated molecules connect with each other using four atoms from each molecule to create a ring with eight atoms. As it is a photochemical reaction, it is triggered by some form of light or radiation.

“The image that shows photo cyclo-addition reaction”


This refers to the chemical reduction of a molecule that takes place in the presence of electromagnetic radiation.


Photolysis is a simple process in which bonds are broken within molecules due to the transfer of light.


Photocatalysis is a process that results in the increase in the rate of a photoreaction by the presence of a catalyst.

Photo Initiator

A photo initiator is a molecule that can create radicals, anions and cations, all reactive species just by the presence of light. 

Common Mistakes

Students often confuse photochemical reactions with thermal reactions, but they are different. Photochemical reactions absorb light energy, whereas thermal reactions involve heat energy.

Contexts and Applications

The topic is very useful for undergraduate and postgraduate courses, especially for Bachelors and Masters in Chemistry.

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