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Question:

In Young's double slit experiment, the separation d between the slits is 2mm, the wavelength λ of light used is 5896oA, and the distance D between the screen and slits is 100cm. It is found that the angular width of the fringes is 0.20o. To increase the angular fringe width to 0.21o (with same λ and D), the separation between the slits needs to be changed to.

2.1mm

1.9mm

1.8mm

1.7mm

Solution:

The correct option is D 1.9mm
Since in YDSE, angular width is given by
Angular Width A.W. = β = λ/d
For a given wavelength,
A.W.1/d1 = A.W.2/d2
0.20 × 2 = 0.21 × d2
d2 = 1.9mm