The main categories of photochemical reactions are as follows:

1. Photosynthesis: This is a special type of photochemical reaction that primarily occurs in plants, algae, and certain bacteria. It uses light energy to convert carbon dioxide and water into organic compounds and oxygen.

2. Photolysis: This refers to the decomposition of a substance under the action of light. For example, water can be decomposed into hydrogen and oxygen under ultraviolet light.

3. Organic Synthesis: Photochemical reactions play a significant role in organic synthesis, especially in situations requiring high selectivity and mild reaction conditions. Key reactions include:

• Photooxidation: This involves the addition reaction of organic molecules with oxygen under light irradiation and the presence of photosensitizers. It is widely used in the synthesis of fine chemicals.

• Photoreduction: This occurs when organic molecules extract hydrogen molecules from a hydrogen donor under photocatalysis.

• Photosubstitution: This involves the substitution of an atom or group in an organic molecule by another atom or group under light irradiation.

4. Environmental Science: Photochemical reactions play a crucial role in the degradation and purification of organic pollutants. Key reactions include photooxidative degradation and photo-microbial degradation.

In summary, photochemical reactions encompass a wide range of categories, each with its unique characteristics and applications.