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which phase difference is not possible

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In Youngs Double Slit Experiment with two identical slits when upper slit is covered with a transparent sheet of mica second order minima observed above the centre of the screen and second order Maxima below it which of the following cannot be possible value of phase difference caused by mica sheet :

1.$ \pi /3$
2. $7\pi /2$
3.$ 10\pi/3$
4. $11\pi/3$

I can't get that which maxima because it can be at $-3\lambda/2 $ or $3\lambda/2$
and even further how shall I decide the phase difference ?

asked Apr 4, 2017 in Physics Problems by physicsapproval (2,320 points)

1 Answer

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Best answer

The mica sheet increases the optical path length from the upper slit to any point on the fringe pattern. The distance from the lower slit to the same point on the fringe pattern must increase in order to compensate, so the fringe pattern moves up the screen.

The 1st order minimum occurs when the phase difference is $pi$, the 2nd order occurs when it is $3\pi$. The 2nd order maximum occurs when the phase difference is $4\pi$.

The 2nd order minimum is now above the centre of the screen, so the phase difference must be greater than $3\pi$. The 2nd order maximum is still below the centre of the screen, so the phase difference must be less than $4\pi$.

The phase difference must be between $3\pi=6\pi/2$ and $4\pi=12\pi/3$. So 2, 3, 4 are possible but not 1.

answered Apr 5, 2017 by sammy gerbil (28,876 points)
selected Apr 5, 2017 by physicsapproval
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