The scientific study of light.
Physical optics is concerned with the creation, nature, and properties of light.
Psychological optics pertains to the role of light in vision. Geometrical optics deals with the properties of reflection and refraction of light, as part of the study of mirrors, lenses, and optical fibers. Although we talk of these categories as if they were separate and distinct, actually the whole reality of sight and vision constantly involves all of these categories in various combinations. For example, consider a rainbow. This visual phenomenon not only involves both reflection and refraction, but also is a practical demonstration that different colors of light refract differently, even under fixed circumstances.