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Photochemistry studies chemical changes initiated or influenced by light. Absorption of a photon can prepare an electronically excited molecule, opening reaction pathways different from those available on its ground-state potential energy surface.^[ref]

The absorbed energy is $E=h\nu$, so changing wavelength changes both which species absorb and which states are reached. Photodissociation of molecular oxygen is an atmospheric example. Absorption alone does not guarantee chemical conversion: relaxation and competing reactions affect the quantum yield. A photon-initiated chain can produce more than one product event per absorbed photon, so a chemical quantum yield need not be at most one.

Photolithography transfers a pattern defined by light into a photosensitive photoresist coating. After exposure and development, the remaining photoresist forms a relief pattern that can protect selected regions during etching or another material-transfer step.^[ref]

A photomask can place bright and dark regions in the projected optical image. A positive photoresist develops more rapidly where exposed, so sufficiently exposed regions are removed; a negative photoresist retains the exposed regions instead. The final geometry depends jointly on the optical image and the material response. In particular, diffraction limits the image delivered by the optics, while photoresist contrast determines how a gradual variation of exposure is converted into a material boundary. Photolithography therefore connects wave optics, photochemistry and semiconductor device fabrication.