Optics Question 152

Question: In Young’s double slit experiment, the two slits act as coherent sources of equal amplitude A and wavelengthl. In another experiment with the same set up the two slits are of equal amplitude A and wavelength l but are incoherent. The ratio of the intensity of light at the mid-point of the screen in the first case to that in the second case is

[IIT-JEE 1986; RPMT 2002]

Options:

A) 1 : 2

B) 2 : 1

C) 4 : 1

D) 1 : 1

Show Answer

Answer:

Correct Answer: B

Solution:

Resultant intensity I=I1+I2+2I1I2cosφ

At central position with coherent source (and I1=I2=I0

Icon=4I0 … (i)

In case of incoherent at a given point, f varies randomly with time so (cos f)av = 0 \ IIncoh=I1+I2=2I0 … (ii)

Hence IcohIIncoh=21 .



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