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Define Plane of Vibration And Plane of Polarisation?
Definition: The plane in which the vibration of the polarised light remains confined is called plane of vibration.
Description: In Fig., the direction of propagation of the polarised light through the tourmaline crystal AB is shown. Light rays are advancing along x -axis and the electric field of polarised light is vibrating along y -axis. xy plane i.e., the plane ABCD is the plane of vibration of polarised light.
Definition: The plane containing the ray of light and perpendicular to the plane of vibration is called plane of polarisation.
In Fig., plane xz i.e., PQRS is the plane of polarisation of light.
- The intermediate angle between plane of vibration and plane of polarisation is 90°.
- Angle between plane of polarisation and direction of propagation of polarised light is zero.
Plane Polarised or Linearly Polarised Light
The vibration of electric field of a light wave on a plane perpendicular to the direction of propagation of an ordinary light or unpolarised light, can take place in any direction from a point.
The direction of vibration in a particular plane perpendicular to the direction of propagation of ray of light, is shown by the arrowheads in different directions in that plane [Fig(a)].
In Fig. as propagation of a transverse vibration takes place along a particular direction (in this case, y -axis) it is a linearly polarised light. Again as the direction of wave propagation and direction of vibration remain confined in a particular plane, so it is also called plane polarised light.
Definition: If vibration of polarised light remains confined in a plane and takes place along a straight line, then it is called plane polarised or linearly polarised light
[Fig.].
Convention of representation of unpolarised and polarised light: In the plane of the paper, unpolarised and polarised lights are represented according to the following conventions.
i) Ordinary light i.e., unpolarised light has vibration in all directions in a plane, perpendicular to the direction of propagation of light [Fig.(a)]. So it is supposed to be made up of two mutually perpendicular vibrations. Hence ordinary unpolarised light is shown with dots and lines with arrows in opposite directions at the same time
[Fig.(b)].
ii) If the polarised light has vibrations in the plane of the paper, it is shown with lines having arrows in opposite directions perpendicular to its direction of propagation [Fig.(c)].
iii) If the vibration of polarised light is in a direction perpendicular to the plane of the paper, they are shown by dots on the line of propagation [Fig.(d)].
1. When unpolarised light is transmitted through an analyser, its intensity is halved.
2. If polarised light is incident on an analyser then the intensity of the transmitted light is given by Malus’ law.
Malus’ law: When a beam of completely plane polarised light is incident on an analyser, the resultant intensity of light (I) transmitted from the analyser varies directly as the square of the cosine of the angle (θ) between plane of transmission of analyser and polariser, i.e.,
I ∝ cos2θ or, I = I0cos2θ
where I0 is the intensity of the light incident on the analyser.