Geometrical optics is a method of describing light in terms of rays. These rays are approximations which are useful in calculating the paths which light propagates in certain situations such as the processes of refraction and reflection.

The simplifying assumptions of geometrical optics include that light rays:

• Propagate in a straight-line as they travel through a uniform medium
• Bend, and in particular circumstances may split in two, at the border between two different media
• Follow curved paths in a medium in which the refractive index changes
• May be absorbed or reflected.