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Light Ray, Light Beam and its different types

Light ray and light beam are the basic terminologies that we frequently use in the study of ray optics or geometrical optics.

Light Ray:

The straight line path along which light travels in homogeneous medium, is called ray. A ray can also be defined as an imaginary straight line drawn in the direction in which light travels.

Light Beam:

Single light ray bears no practical significance as no real light source can be there that emits just a single ray of light. There is always a bunch of light rays travelling together.

A collection of light rays travelling together is known as a light beam.

Types of Light Beam:

There are mainly three types of beam as discussed below:

1.) Parallel beam of light: It is a bundle of light rays which are parallel to one another as shown below in the diagram. Diameter of the beam remains same everywhere.

Practically, you will not find a source of light or an illuminated structure emitting a 100% parallel beam of light. Ideally a large and uniform planar source of light can be a source of a perfectly parallel beam.

If the source of light is located at very large distance from the region of observation (like the sun), then in that case also the beam obtained is effectively parallel (for smaller regions only).

2.) Divergent beam of light: It is a beam of light in which all the rays meet at a point when produced backward. Diameter of such beam goes on increasing as the rays proceed forward. All the physical light sources generally emit divergent beam.

The rays going out from a point source of light is the most relevant example of a divergent beam.

3.) Convergent beam of light: It is the beam of light in which the rays meet (or converge) at a point and the diameter of the beam goes on decreasing in the direction of rays.

A parallel beam of light after passing through a convergent lens becomes a convergent beam. A real light source does not emit a converging beam directly.

Validity of Ray Approximation of Light?

The ray or beam approximation of light is not valid or relevant for all the cases. Light (or visible light) is a narrow band of electromagnetic waves (spectrum). The ray approximation holds good only when the size of the obstacle that light encounters is much greater than the wavelength of light. Under such cases, the wave nature is not dominant and the path of light rays can be approximated as straight lines.

At macroscopic levels, we generally treat the light as rays or beams to deal with the study of simple facts like rectilinear propagation, laws of reflection and refraction, image formation by geometrical methods etc.