The answer is D, why? 14. The diagram below shows the polarising angles of three

ID: 1393255 • Letter: T

Question

The answer is D, why?

14. The diagram below shows the polarising angles of three polarising sheets. Unpolarise light traveling in the +z direction impinges on sheet 1. and then transmits through tw more sheets as shown, where sheet 2 has a polarisation angle theta with respect to that o sheet 1, and sheet 3 has a polarization angle at 90 degree to the polarisation angle of sheet 1 Which of the following is incorrect? A. the intensity after sheet 3 can increase when sheet 2 is inserted B. the intensity after sheet 1 is half the incident light intensity C. the intensity after sheet 3 is zero if theta = 0 degree or theta = 90 degree D. the intensity after sheet 3 is always zero no matter what the value of theta E. the intensity depends on the square of the wave amplitude

Explanation / Answer

A) When a second polarizer is rotated, the vector component perpendicular to its transmission plane is absorbed, reducing its amplitude to E0 cos(theta) (keeping only the component perpendicular to the plane of polarization).

Now when the third polariser is inserted, the new total intesity is: E0 cos(theta) cos(90 - theta).................(1)

and intensity will be its square.

without the sedond polarizer, the two polarizer would have been in cross states leading to zero intesity.

B) in unpolarized, we can consider that half the intensity is in horizontal polarization and half in the vertical, when a vertical polarizer is inserted, only the vertical intensity is left that is half of initial intensity.

we can also solve it using Malus' law : the intensity of the transmitted light through a polarizer kept at an angle theta is: ... I = I*[cos^2 (theta)]

For an Unpolarized initial beam there are many linear polarization directions which are randomly oriented and so they have an average cos^2 value of 1/2.

(C) you can infer this from eqn (1) very easily.

(D) wrong (contradicting C)

(E) well known fact :)