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JEE Advanced · Physics · 25. Wave Optics

In a Young's double slit experiment, the slit separation d is 0.3 mm and the screen distance D is 1m. A parallel beam of light of wavelength 600 nm is incident on the slits at angle α as shown in figure. On the screen, the point O is equidistant from the slits and distance PO is 11.0 mm. Which of the following statement(s) is/are correct?

  1. A For α=0.36π degree, there will be destructive interference at point O.
  2. B Fringe spacing depends on α
  3. C For α=0.36π degree, there will be destructive interference at point P
  4. D For α=0, there will be constructive interference at point P.
Verified Solution

Answer & Solution

Correct Answer

(C) For α=0.36π degree, there will be destructive interference at point P

Step-by-step Solution

Detailed explanation


Path difference at P , x=dsinα+dsinθ
θ, α: small angle,sinθtanθ=yD and sinαα
x=dα+dyD
A x=3 mm×0.36π×π180+0=600 nm
600 nm=nλ
n=1
Constructive interference A is incorrect
B Fringe width does not depend on α.B is incorrect
\(\text{(C) }\Delta x=0.3\text{ mm} \times \frac{0.36}{\pi} \times \frac{\pi}{180}+\frac{0.3\text{ mm} \times 11\text{ mm}}{1000}\) \(=39 \times 10^{-4}\text{ mm}\)
Now, x=39×10-4=2n-1×600×10-9×1032
n=7
Destructive interference happened correct
(D) \(\alpha=0 \therefore \Delta x=d y / D=\frac{0.3 \times 11}{1000}=33\) \(\times ~10^{-4}\text{ mm}\)
x in terms of λ=33×10-4600×10-6λ=11λ2
as x=2n-1λ2
There will be destructive interference. D is incorrect.
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