# Definitions

## Definitions

Definitions

### Ray Ray
In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing.

### lens lens
An object, usually made of glass, that focuses or defocuses the light that passes through it.

### focal point focal point
(optics) A focus; a point at which rays of light or other radiation converge.

### focal length focal length
The focal length of an optical system is a measure of how strongly it converges (focuses) or diverges (diffuses) light. A system with a shorter focal length has greater optical power than one with a long focal length.

### principle (optical) axis principle (optical) axis
In an optical system, the optical axis is an imaginary line that defines the path along which light propagates through the system. For a system composed of simple lenses and mirrors, the axis passes through the center of curvature of each surface, and coincides with the axis of rotational symmetry. The optical axis is often coincident with the system's mechanical axis, but not always, as in the case of off-axis optical systems.

### focal plane focal plane
The front and rear (or back) focal planes are defined as the planes, perpendicular to the optic axis, which pass through the front and rear focal points. An object an infinite distance away from the optical system forms an image at the rear focal plane. For objects a finite distance away, the image is formed at a different location, but rays that leave the object parallel to one another cross at the rear focal plane.

### converging lens converging lens
If the lens is biconvex or plano-convex, a collimated or parallel beam of light travelling parallel to the lens axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens (known as the focal length). In this case, the lens is called a positive or converging lens.

### diverging lens diverging lens
If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens.

### real image real image
In optics, a real image is a representation of an actual object (source) formed by rays of light passing through the image. If a screen is placed in the plane of a real image the image will generally become visible. The image seen on a cinema screen is an example of a real image.

### virtual image virtual image
In optics, a virtual image is an image in which the outgoing rays from a point on the object never actually intersect at a point. A simple example is a flat mirror where the image of oneself is perceived at twice the distance from yourself to the mirror. That is, if you are half a meter in front of the mirror, your image will appear at a distance of half a meter inside or behind the mirror.

### upright image upright image
Content=My content goes here

### inverted image inverted image
Content=My content goes here