Light is an electromagnetic wave with electric and magnetic field vectors varying sinusoidally, perpendicular to each other as well as perpendicular to the direction of propagation of wave of light.
That is to say electromagnetic waves are transverse in nature. The following experiment helps us understand this nature of light. But let us consider it for a transverse wave produced in a string. A and B are two cardboards with a through slit cut as shown. Pass a thin string through both the slits and vibrate the string in all directions. The arrows indicate that the vibration of the string occurs in all directions. On passing through A, only the vertical vibration of the string appears and these continue to go through B and come out finally.
When the slit of board B is placed perpendicular to A, no vibrations appear to come out of B, although vibrations appear in the region between A and B. We therefore say that in the former case the waves are polarized. Polarization is the phenomenon exhibited by a transverse wave, wherein the vibrations of particles in such waves are restricted to a single plane. The boards A and B are called polarizer and analyzer respectively. In this mechanical model, if we replace A and B with two tourmaline crystals and light waves instead of a string, one can expect light waves to be polarized. This is explained as follows. Since atoms in a light source act independently, light propagated from such waves have their planes of vibrations randomly oriented as shown. Such light is called unpolarised.
The field vectors i.e., can be resolved into y and z components. The unpolarised light can be thought of as superposition of two polarized waves whose planes of vibration are perpendicular to each other as in fig (b).
This in turn is further simplified to represent as in Fig (c). The arrow represents vibration of field vector parallel to a plane of paper and the dot (.) That field vector is perpendicular to the plane of paper. Therefore remember is a simpler way to visualize an unpolarised light wave.
Experiment to prove transverse nature of light waves:
When unpolarised light is passed through a tourmaline crystal cut with its face parallel to its axis AB, only those vibrations of light pass through which are parallel to AB and all others are absorbed. The light intensity, therefore, is reduced. The emergent light from the first crystal is a plane-polarized light. This can be checked by using a second crystal. When this second crystal is rotated, a change in intensity is noted, the light is said to be plane polarized. When the axis of the second crystal is perpendicular to the axis of the first crystal, no light is passed. The light coming out of the crystal T1 is said to be polarized i.e., the vibrations of light (electric vector) is restricted in a particular direction. This phenomenon is called polarization. The first crystal acts as a polarizer and the second crystal acts as an analyzer.
i.e., T2 analyses if the light waves are polarized or not.
Note: Longitudinal waves cannot be polarized. Can you guess why?
The above experiment confirms that light waves are transverse in nature.
can be resolved into y and z components. The unpolarised light can be thought of as superposition of two polarized waves whose planes of vibration are perpendicular to each other as in fig (b).
represents vibration of field vector parallel to a plane of paper and the dot (.) That field vector is perpendicular to the plane of paper. Therefore remember 
is a simpler way to visualize an unpolarised light wave.
