Photochemistry

Introduction: Photons are often employed to energise chemical reactions. Usually radiation of well-defined energy is employed for this purpose. The energy or the frequency of radiation depends on the strength of the bond that needs to be broken in the reaction.

Photochemistry deals with the absorption, excitation and emission of photons by atoms, ions, molecules etc. It is a branch of chemistry and is the study of chemical reactions that occur with the absorption of light by atoms or molecules. Photochemistry is basically the study of photochemical reactions.

 Some reactions do not take place in the dark but take place in the presence of light or some other radiation. Such reactions are called photochemical reactions. Examples include:

• Photosynthesis of carbohydrates in plants
• Formation and dissociation of ozone in the stratosphere
• Vision involves a photochemical reaction in which the compound, retinal, present in the eye undergoes isomerisation by absorbing the light falling on it.
• Absorption and subsequent emission of a photon by a gas phase atom such as sodium.

All these reactions occur as light is a form of energy and it provides the necessary activation energy for the reaction to take place.

Rates of photochemical reactions depend on the intensity of the radiation. An important feature of a photochemical experiment is the measurement of the intensity of radiation. The unit of photochemical intensity is Einstein. Temperature has a very little effect on the rate of a photochemical reaction and instead the intensity of light has marked effect on the rate of photochemical reaction.

Role of Photochemistry in everyday life: Photochemistry plays an important role in everyday life.

 A few examples are given below:

• In vision
• In photosynthesis
• In the manufacture of many organic compounds. E.g a number of polymerisation reactions are initiated by light.
• In photography
• In modern printing technology
• In photo-etching used in electronic industry.
• In the manufacture of integrated circuits used in electronic devices.

Damaging effects of photochemistry: The most well known damaging effect of photochemistry is the effect of sunlight on the dyes used for colouring of fabrics. Due to this effect, coloured fabrics fade away.