Some Important Terms Related with Light
The waves which do not require a material medium for their propagation are known as electromagnetic waves. e.g. all light waves are electromagnetic waves.
SOURCE OF LIGHT
A source of light is an object, from which light is given out. The source of light are of two kinds:
(i) Luminous source of light: A luminous source of light is that which emits light itself. For example : The sun, the stars, etc.
(ii) Non Luminous source of Light : The source of light which does not emit light itself is called a non-luminous source of light.
For example, planets, moon, building, trees etc.
A medium is a substance through which light propagates or tries to do so.
(i) Transparent : The medium which allows most of the light to pass through it is called a transparent medium. For example : air, water, glass etc.
(ii) Translucent : The medium which allows only a part of the light to pass through it is called a translucent medium. For example : paper, ground glass etc.
(iii) Opaque: The medium which does not allow any light to pass through it is called opaque medium. For example : wood, bricks, metals etc.
A ray of light is the straight line path along which light travels. It is represented by an arrow head, on a straight line . The direction of arrow gives the direction of propagation of light.
BEAM OF LIGHT
A group of parallel rays is called a beam of light. It is also called a pencil of light.
A pencil of light may be of three types.
(i) Convergent pencil
(ii) Divergent Pencil
(iii) Parallel Pencil
Convergent pencil is also known as convergent beam of light. In the convergent pencil, as the beam of light progresses the rays converge to a point.
A convergent Pencil is that in which rays of light propagates to meet at a particular point say O as shown in the figure below.
The diameter of the pencil goes on decreasing as the rays proceed forward.
Divergent pencil is also known as divergent beam of light. In the divergent pencil, as the beam of light progresses, the rays diverge from each other.
A divergent pencil is that in which rays of light come out of a point source such that the diameter of the pencil goes on increasing as the rays proceed forward as shown in the figure below.
Parallel pencil is that in which all the rays move parallel to one another. The diameter of the pencil remains constant throughout as shown in the figure below.