A light beam is light which propagates essentially in one direction (directional light) and at the same time has a limited spatial extension in directions perpendicular to its beam direction. Often, it has a roughly circular cross section but no precise spatial limitation.
A plane wave also propagates in a certain direction but does not have the limited transverse spatial extension, and is therefore not regarded as a light beam.
Light beams are occasionally encountered in nature, but play many import roles in technical optics and photonics – in particular, in the form of laser beams. Those are most suitable for applications like free-space optical communications, imaging or material processing. Many optical techniques and devices work only with light beams, not with light of other forms.
Light beams are generally considered in homogeneous transparent media – for example, as free-space light beams in air or vacuum. However, their character may be preserved e.g. when hitting a mirror or a prism which essentially just sends the light into a new direction. They may also be manipulated with other types of optical elements – for example, with lenses for refocusing, optical filters for modifying their spectral characteristics, optical modulators for modulating properties like optical power and polarization, and beam splitters for splitting them into two or more beams.
A light beam can be seen by an eye outside its covered volume only if part of the light is scattered towards that light – for example, by dust particles on density fluctuations in air. Generally, a light detector can only detect light entering it, but not just passing it.
In contrast to light beams, light rays are a more abstract concept used within ray optics, also called geometrical optics. They have no transverse extension. Light beams are sometimes described as bundles of rays.
Properties of Light Beams
Specific properties of light beams are spatial properties, in addition to other properties of light which are encountered also for other forms of light. Examples for the latter are optical frequency and optical bandwidth, optical power and polarization. Some of those properties may change along a beam; for example, the optical power may be gradually reduced by absorption or scattering in the medium.
A light beam with circular cross section may be characterized with a beam radius which generally changes during propagation. Over long enough distances, it must increase due to diffraction, since light is a wave phenomenon (→ wave optics). However, the beam radius may initially decrease before reaching a beam focus (or beam waist) where it has its minimum value. The increase in beam radius in the far field is associated with the beam divergence. A light beam can actually have two different focus positions e.g. for x and y direction; this is called astigmatism.
A special kind of light beams are Gaussian beams, having a Gaussian amplitude profile with flat or parabolic phase profile. In contrast to those, multimode beams have more complicated spatial properties.
Generation of Light Beams
Non-laser light sources (e.g. incandescent lamps or gas discharge lamps) usually emit light in a spatial form which is not a beam. However, one can form light beams from such light. For example, one may place the filament of an incandescent lamp in the focus of a parabolic mirror in order to collimate the light (or at least some substantial fraction of its total power), i.e., send it to essential one direction, which e.g. allows for more controlled illumination of some object.
A remarkable difference between laser beams and non-laser beams is that the former usually exhibit a far higher beam quality. Besides, many other properties of laser light differ much from light made with other sources; see the article on laser light for details.
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