First slide

Coherent sources and their importance in interference

Question

Two beams of light having intensities I and 4I interfere to produce a fringe pattern on a screen. The phase difference between the beams is $\frac{π}{2}$ at point A and $π$ at point B. Then the difference between the resultant intensities at A and B is

Easy

A

2I
B

4I
C

5I
D

7I

Solution

At point A, resultant intensity
$I_{A} = I_{1} + I_{2} = 5 I; and at point B I_{B} = I_{1} + I_{2} + 2 \sqrt{I_{1} I_{2}} \cos π = 5 I - 4 I I_{B} = I so I_{A} - I_{B} = 5 I - I = 4 I$

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