Many times reactions go with heat: as you heat molecules, they start to move around and so hit other molecules which can then react. What molecules need is just some activation energy, a little energy to start the reaction. In the case of photochemistry, this energy is given by the light. A molecule or an atom absorbs at a certain wavelength. If light is given at this wavelength then the molecule or atom becomes excited. It is then easier for it to react.
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