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Two light beams of intensities I and 4I interfere on a screen. At point A the phase difference between them is pi/2, and at point B it is pi. What is the difference between the resultant intensities at A and B?

1. 2I
2. 4I
3. 5I
4. 7I

Correct answer: 4I

Solution

Resultant intensity: I_res = I + 4I + 2*sqrt(I*4I)*cos(phi) = 5I + 4I*cos(phi). At A (phi = pi/2): I_A = 5I + 0 = 5I. At B (phi = pi): I_B = 5I - 4I = I. Difference = 5I - I = 4I.

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