Name: Waves and Their Phases
Uploaded: 2019-07-19
Duration: 6 min 51 s
Description: In this video, we will learn how two or more waves can be studied in terms of their phases. Equations of two waves and graphs of two waves. Learn more!

Question

Define plane of vibration and plane of polarisation. Give explanation of Malus law and explain parallel and crossed arrangement?

Accepted Answer

The plane that consists of direction of vibration and direction of propagation of wave is known as plane of vibration. In the above diagram, ABCD is the plane of vibration.

Plane of polarisation is perpendicular to plane of vibration. Thus, EFGH is the plane of polarisation.

Polaroid is a cheap and simple method to get plane polarised light. It consists of a large-sized film which is kept between two glass plates. This film is prepared by very small sized crystals of quinine or sulphate and are fixed on the sheet of nitro-cellulose in such a way that their optic axes may remain parallel to each other. Every Polaroid has its characteristic direction which is called Polarising direction.

According to Malus Law, when an unpolarised light is transmitted through polariser and analyser, then intensity, $I$ of light is directly proportional to the cosine of angle between the transmission axis of analyser and polariser. If $I_{o}$ be the maximum intensity of incident light on the Polaroid and $θ$ is the angle between polariser and analyser then ,

$I \propto \cos^{2} θ \Rightarrow I = I_{0} \cos^{2} θ$

It is called law of Malus.

When both Polaroids are in parallel arrangement, then

$θ = 0 \Rightarrow I = I_{0} \cos^{2} 0 ° \Rightarrow I = I_{0}$

which is the maximum intensity.

When Polaroids are in crossed arrangement,

$θ = 90 ° \Rightarrow I = I_{0} \cos^{2} 90^{°} \Rightarrow I = 0$

which is the minimum intensity.

Define plane of vibration and plane of polarisation. Give explanation of Malus law and explain parallel and crossed arrangement?

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