Geometrical optics

Geometrical optics models light as rays that travel along paths and change direction at optical surfaces. It describes reflection, refraction, and image formation while neglecting wave effects such as diffraction and interference.

Fermat's principle: A principle stating that light follows paths for which travel time is stationary relative to nearby paths. It provides a variational rule from which reflection and refraction laws can be derived.
Light ray: An idealized line showing the direction of light propagation in an optical model. Geometrical optics represents propagation with these directed paths.
Lens: An optical element with refracting surfaces that converges or diverges transmitted light. Its focal length and shape determine how it forms images from rays.
Physical optics: The study of optical phenomena that require wave behavior, including diffraction and interference. It handles effects that ray-based geometrical optics deliberately omits.
Law of reflection: The rule that a reflected ray makes equal angles with the surface normal as the incident ray. It determines how rays change direction at mirrors.
Refractive index: A dimensionless measure of how much light slows in a medium relative to vacuum. Its changes across boundaries determine refraction and optical path length.
Mirror: A surface that reflects light, often used to redirect or focus rays. Geometrical optics predicts image formation by plane and curved mirrors.
Diffraction: The spreading and redistribution of waves when they encounter apertures, obstacles, or edges. It sets limits on ray predictions near edges and small openings.
Snell's law: The relation between incident and refracted angles, refractive indices, and the surface normal. It gives the ray direction after light crosses between media.
Optical axis: A reference line of symmetry through an optical system or one of its elements. It organizes ray geometry in centered lenses and mirrors.

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