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What is Meant by Polarisation of Light? What is Polariser and Analyser?
The phenomena of interference and diffraction demonstrate that light propagates in the form of waves. But it is not understandable from interference and diffraction whether the light waves are transverse or longitudinal in nature because both longitudinal and transverse waves exhibit interference and diffraction.
The topic of discussion of this section is polarisation of light. This phenomenon of light distinctly proves that light waves are transverse in nature and not longitudinal like sound waves.
Polarisation of mechanical Waves: Two narrow slits A and B are cut in the middle portions of two cardboards C1 and C2 [Fig.]. A thin long string OE, tied at one end E to a rigid support is passed through the slits A and B.
Now, holding the end O of the string, it is made to vibrate perpendicularly to OE. As a result, a transverse wave advances along the direction OE. If x-axis is taken along OE, the parti-cles of the string will vibrate perpendicular to x -axis, i.e., in the yz plane. Holding the end O, the string can be made to vibrate randomly i.e., in any direction in the yz plane. In that case, each particle of the string, in the intermediate portion of the string between O and A, will have two vertical components of transverse vibration along the y and z -axes [Fig.].
At first the cardboards C1 and C2 are so placed that both the slits A and B are parallel to y-axis [Fig.]. Clearly, the z – component of vibration will be obstructed by the slit A, but the y – component will pass through A without any obstruction and reach the section AB. So inspite of random vibrations of the string in the portion OA, the vibration of the string in the section AB will be confined only along y -axis. This phenomenon of converting the random vibrations of a transverse wave to uni-directional vibration is called polarisation.
In this case the transverse wave in the section OA is unpolarised, but it turns into polarised wave in the section A B by the slit A because wave of this section (section AB) vibrates only along y -axis. Since the slit B is parallel to y -axis, so the vibration of the wave along y -axis in the section AB, will pass through the slit B also without any obstruction. Thus, the transverse wave will propagate upto point E [Fig.(a)].
Now if the slit B is rotated through 90° with respect to OE, then the slit becomes parallel to z -axis [Fig.]. Clearly, the vibrations of the string of the section AB along y -axis, get completely obstructed by the slit B. So, no vibration exists in the section BE of the string i.e., the transverse wave cannot propagate along BE [Fig.].
From the above discussion, it is clear that if the vibration is longitudinal, that is, parallel to x -axis, then, they are not at all obstructed by slits A and B in any of their orientations. Thus the longitudinal wave can propagate up to point E. Hence, it can be said that polarisation is a phenomenon which is not exhibited by longitudinal wave. For example, sound wave is a longitudinal wave, hence sound wave is not polarised.
Unpolorised Light: In the usual sources of light like the sun, candle, electric lamp, etc. electrons, ions or other charged particles vibrate randomly. Hence the transverse vibrations of the waves emitted from these sources have no definite direction. This type of light is called ordinary light or unpolarised light [section OA of Fig.]. In this case, the transverse vibration may be referred to as the sum of the two perpendicular components of equal amplitude.
Light is an electromagnetic wave. The electric field \(\vec{E}\) and the magnetic field \(\vec{B}\) of this wave always vibrate perpendicular to the direction of the wave. The vibration is confined to a certain plane. The wave propagates in a direction perpendicular to the plane.
Definition: The phenomenon of restricting the vibrations of the electric vector of a light wave along a particular axis in a plane perpendicular to the direction of the light wave is called polarisation of light.
The polarisation of light can be easily explained by an experiment with tourmaline crystal.
Experiment with a tourmaline crystal: Tourmaline is a hexagonal crystal [Fig.]. The crystal cut in the form of a thin plate behaves almost as a transparent substance. The longest diagonal of the hexagonal crystal is called the crystallographic axis or optical axis. In Fig., C1 and C2 are two thin tourma line crystal sheets and M1N1 and M2N2 are their optical axes respectively.
O is an ordinary source of light. Keeping the eye fixed at position E, one is looking towards O. Here x -axis is taken along OE. At first, the crystal C1 is placed on the way of the ray OE at a location A in such a way that its optical axis M1N1 lies perpendicular to x -axis.
If the crystal is so placed, the intensity of light is found to be a little diminished. If the crystal is made to rotate about OE as the axis of rotation, the intensity of the transmitted light remains unchanged.
Now the crystal C2 is also placed at B on the way of the ray OE in such a way that the optic axes of both C1 and C2 are parallel to y -axis [Fig.(a)]. It is found that light comes out undiminished in intensity inspite of C2 being placed.
But as the crystal C2 is rotated slowly about the point B with OE as the axis of rotation, it is found that the intensity of light goes on decreasing. When the axis of C2 makes an angle 90° with the axis of C1 i.e., crystallographic axis M2N2 becomes parallel to z -axis, no light from the source reaches the eye [Fig.(b)]. When C2 is rotated further, the intensity of the light gradually increases. When C2 is rotated through another 90°, i.e., it is rotated through 180°, from its initial position, light reappears with its earlier intensity.
Explanation of the result of the experiment: The above experiment can be explained if we consider light waves as transverse in nature and the crystallographic axes of the tourma-line crystals as narrow slits. The transverse vibrations of the light waves emitted from the source O are random in nature. So, two perpendicular components of vibration along y and z -axes exist in each point of the section OA of the light ray [Fig.].
Since the axis M1N1 of the crystal C1 is placed parallel to y -axis, so the y -component of the transverse vibration of the light wave passes through the crystal, but the z -component is completely absorbed. Since one component is absorbed completely, the intensity of the transmitted light becomes half. Only y -component of the transverse vibration of the light wave has been shown in the section AB.
Now if the crystallographic axis M2N2 of the crystal C2 also becomes parallel to y -axis, the y -component of the transverse vibration passes through the crystal and reaches the eyes [Fig.(a)). But by rotating the optical axis M2N2 through 90°, if it is placed parallel to z -axis, the crystal C2 absorbs the y – component of the vibration completely.
So, no vibration exists in the section BE; light wave is absent here. Hence no light reaches the eye [Fig.(b)]. When crystal C2 is rotated through another 90° i.e., when total rotation is 180°, the optical axis M2N2 becomes parallel to y -axis again, as a result the y – component of the transverse vibration can pass through the crystal C2.
Conclusion:
i) When an ordinary light wave passes through a tourmaline crystal or a similar medium, its random transverse vibrations are converted to an unidirectional transverse vibration. This phenomenon is called polarisation of light and the light is called polarised light. In the [Fig.(b)], the light of the section AB is called polarised light.
ii) Any transverse wave, like light wave can be polarised.
Polariser: The instrument by which unpolarised light is made polarised is called polariser. The tourmaline crystal C1 is called polariser and the crystallographic axis M1N1 is called polarising axis.
Analyser: The instrument which examines whether light is polarised or not and the type of polarisation, is called analyser. The tourmaline crystal C2 is called analyser because it examines whether the light is polarised or not and what type of polarisation has been produced by the crystal C1. When the crystallographic axes of the crystals C1 and C2 are parallel, it is called the parallel position of polariser and analyser. When their crystallographic axes are perpendicular to each other, they are said to be in crossed position.