# Geometrical optics

Model of optics that describes light propagation in terms of rays.

- Geometrical optics105 related topics

## Focus (optics)

In geometrical optics, a focus, also called an image point, is a point where light rays originating from a point on the object converge.

## Refractive index

Optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.

Since the refractive index of the ionosphere (a plasma), is less than unity, electromagnetic waves propagating through the plasma are bent "away from the normal" (see Geometric optics) allowing the radio wave to be refracted back toward earth, thus enabling long-distance radio communications.

## Mirror image

Reflected duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface.

In physics, mirror images are investigated in the subject called geometrical optics.

## Optics

Branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it.

The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces.

## Optical aberration

Property of optical systems, such as lenses, that causes light to be spread out over some region of space rather than focused to a point.

Aberration can be analyzed with the techniques of geometrical optics.

## Ray (optics)

Idealized geometrical model of light, obtained by choosing a curve that is perpendicular to the wavefronts of the actual light, and that points in the direction of energy flow.

Ray optics or geometrical optics does not describe phenomena such as diffraction, which require wave optics theory.

## Fermat's principle

Fermat's principle, also known as the principle of least time, is the link between ray optics and wave optics.

## Paraxial approximation

In geometric optics, the paraxial approximation is a small-angle approximation used in Gaussian optics and ray tracing of light through an optical system (such as a lens).

## Gaussian optics

Gaussian optics is a technique in geometrical optics that describes the behaviour of light rays in optical systems by using the paraxial approximation, in which only rays which make small angles with the optical axis of the system are considered.

## Paradigm shift

Fundamental change in the basic concepts and experimental practices of a.

The transition in optics from geometrical optics to physical optics with Augustin-Jean Fresnel's wave theory.