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11 Mar 2010 15:38:13 IST

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Two Polaroids are set in crossed positions.A third polaroid is placed between the two making an angl

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Two Polaroids are set in crossed positions.A third polaroid is placed between the two making an angle (theetha) with the passes axis of first polaroid.Write the intensity of light transmitted from second polaroid ????

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edison

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11 Mar 2010 15:58:01 IST

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Here we assume that the light passing through the first polaroid is polarized, thus we use Malu's law for intensity obtained through the polaroid

It is the law giving the intensity produced when a polarizer is placed in front of an incident beam.

Thus if incident light intensity = I₀

Then light through 1st polaroid if kept parallel to the incident plane of polarisation = I₀ cos² 0 = I0

SImilarly the light through sandwiched polaroid (3rd polaroid) = I₀ cos² 45

and light through second polaroid = I₀ cos2 45cos2 45 = I₀ cos⁴ 45 = I₀/ 4

Abhijeet Srivastava

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11 Mar 2010 15:58:52 IST

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Sir,ANSWER ????????

Venkatesh Ganesan

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20 Mar 2010 01:33:29 IST

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This is a sum involving application of Malus Law.

Also, I have replaced 'theeta' with 'x' for simplicity.

Let's first take the case of the unpolarised light (of intensity I) being incident on the first polaroid.

In this case the intensity of transmitted light through the first polaroid is = I/2.

Now, the third polaroid is placed at an angle 'x' with the first, so intensity of transmitted light through this third polaroid is = (I/2)cos^2(x)....................according to Malus Law.

The second polaroid now comes after the third as per the question. We know that the third polaroid is at an angle 'x' with the first polaroid. Also, we know that the first and second polaroids are crossed, ie, they are at an angle 90 degree with each other.

Therefore angle made by the third polaroid with the second polaroid is = (90-x).

Intensity of transmitted light through the second polaroid

= (Intensity of light through 3rd polaroid)*cos^2(90-x)

= (I/2)cos^2(x)sin^2(x).

That's the answer:)

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