Polarization of Light:
During propagation of light, the particles of the medium do not vibrate i.e., light waves are not mechanical waves. It has been proved both theoretically and experimentally that light waves are electromagnetic waves.
In electromagnetic waves, electric and magnetic fields, which are at right angles to each other, vibrate perpendicular to the direction of propagation of the wave i.e., if an electromagnetic wave is traveling along X-axis, the electric field E vibrates along Y-axis, and the magnetic field B along Z-axis. In most of the discussions, the study of electric vector E is sufficient, because its specification automatically fixes the corresponding magnetic vector B. Thus, in a light wave, the vibrating quantity that concerns most is its electric vector E.
Such a wave is also called a plane polarized (or polarized) wave because the entire wave is contained in one plane as shown in figure (a). A linearly polarized wave is usually represented by a double arrow indicating the two equal and oppositely directed maximum values of E as shown in figure (b).
The electromagnetic wave emitted by the antenna is linearly polarized. The light pulses emitted by individual atoms are also linearly polarized but the waves given out by an ordinary source of light are not polarized. This is because an ordinary source of light contains an enormous number of atoms and molecules oriented in a random manner. Consequently, the light given out by such a source in a given direction consists of independent wave trains whose electric vectors E, though perpendicular to the direction of propagation of light, are randomly oriented i.e. if the wave trains are propagating along X-axis, the vibrating electric vectors E are randomly oriented in the Y-Z plane as shown below.
Light emitted by an ordinary source consists of a broken chain of wave trains whose vibrating electric vectors, though right angles to the direction of propagation, are not necessarily parallel to each other, but are randomly oriented in a plane perpendicular to the direction of propagation of light- such a light is called unpolarized light.
Any vibration in an unpolarized light can be decomposed into two mutually perpendicular components. For example, the vibrations shown in figure (d) can be resolved into two components, one along Y-direction and the other long Z-direction. An unpolarized beam of light is, therefore, represented as shown in figure (e) in which vertical double arrows show vibrations in the plane of the paper and dots represent vibration perpendicular to the plane of the paper.
The random orientation of vibration produces symmetry in the direction of propagation, hence truly transverse nature of light remains concealed unless the experimental results are carefully analyzed.
The polarization of light can be demonstrated experimentally by simple arrangement as shown.
Allow the unpolarized light to fall on tourmaline crystal T1. Now the light coming out of T1 is linearly polarized. To check, whether the light coming out of T1, has been polarized or not, we allow the light to pass through another tourmaline crystal T2.
When the axis of T1 and T2 is parallel to each other, then we observe that light passes through both T1 and T2.
And when the axis of T1 and T2 are perpendicular to each other, then we observe that T2 does not allow any light to pass through it as shown. The crystal T1, which polarizes the light is called a polarizer, and the tourmaline crystal T2, which analyses whether the given light is polarized or not is called Analyzer.
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