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What is an unpolarised light ? Explain with the help of suitable ray diagram how an unpolarised light can be polarised by reflected from a transparent medium. Write the expression for Brewster angle in terms of the refractive index of denser medium. |
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Answer» SOLUTION :Ordinary light propagating in a GIVEN direction consists of many independent waves whose planes of vibration are randomly oriented in all directions in a transverse plane. Such light is said to be unpolarised light. In an unpolarised light the electric vector takes all possible directions is the transverse plane. Plane polarised light can be obtained by reflection from a transparent surface (say a plane glass plate) when the light is incident on the surface at the polarising angle `i_(p)`. In such a CONDITION, the reflected light contains vibrations of electric vector PERPENDICULAR to the plane of incidence only and is, thus, completely plane polarised one. when light is incident at the polarising angle `i_(p)`, the reflected and refracted rays make a right angle with each other i.e., `angler=angle(90^(@)-i_(p))`. therefore, as per Snell.s law, we have `n=(sini_(p))/(sinr)=(sini_(p))/(sin(90^(@)-i_(p)))=(sini_(p))/(cosi_(p))=tani_(p)` The condition `n=tani_(p)` is referred as Brewster.s law.  .
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