Lord Maxwell explained the nature of displacement associated with light wave. In his well known electromagnetic theory he points out that light waves are electromagnetic which consists of a vibrating electric and magnetic field. Both the fields are mutually perpendicular to each other and both are perpendicular to the direction of propagation as shown in Figure 6.3.
In the figure E represents the electric field vector and B represents the magnetic field vector. It is the electric field vector which produces all the observed effect of light. So, vibration mean vibrations of electric field vector.
In any light source, light is emitted by atoms or molecules. As source of light contains million atoms and million excited atoms oriented at random which emit light for a very short duration. Thus ordinary light will have vibrations in all directions perpendicular to the direction of propagation. The unpolarised light is generally represented by a star *·
It is to be noted that a vibration which is in plane of the paper, is represented by a double head arrow. In contrary to it, if the vibration is perpendicular to the plane of paper, it is then represented by a dot.