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Why does the intensity of unpolarized light remain unchanged when it pass through a quarter wave plate? A quarter wave plate produces a phase difference between e- ray and o-ray. But the intensity is changed for plane polarized light.

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An ideal quarter wave plate does not absorb any light.
The speed of light through the quarter wave depends on the orientation of the electric field vector relative to the plate. So one orientation is slowed down more than the other and the thickness of the plate is designed so that there is a change of phase of $90^\circ$ for one plane relative to another.
This represents a change of phase equivalent to a quarter of a wavelength.

An ordinary Polaroid actually absorbs light.
The light which emerges has the electric field oscillating in one plane the plane at right angles having been absorbed.

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  • $\begingroup$ Thnkzz..and..why is d unpolarised is not polarised when it pass through quarter wave plate...quarter wave plate produce path difference between e-ray and -oray....and also both of them are plane polarised @farcher $\endgroup$ – Izzaki Ningombam Chester Oct 1 '16 at 17:04
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    $\begingroup$ It is because there is no absorption. All that is different is the speed of the light passing through the plate. $\endgroup$ – Farcher Oct 1 '16 at 19:05
  • $\begingroup$ Then.....when a polarised light is passed through a quarter wave plate....there is absorption of light???@farcher $\endgroup$ – Izzaki Ningombam Chester Oct 2 '16 at 5:25
  • $\begingroup$ I do not understand your question. $\endgroup$ – Farcher Oct 2 '16 at 6:01
  • $\begingroup$ You said to me that Unpolarised light is not polarised when it pas through a quarter wave plate coz there is no abdorption of light....then my question is...when a plane polarised light is passed through a quarter wave plate....is there is absorbtion of light??? $\endgroup$ – Izzaki Ningombam Chester Oct 2 '16 at 7:38
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The intensity shouldn't change for either polarized or unpolarized if neglecting the reflections and absorption caused by the quarter waveplate.

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Because quarter wave plate does not absorb any light. The phase differences is between the electric field signals in the x and y direction. A change in phase like that can change the polarisation into linear/circular/elliptical.

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