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A Young's double slit set-up for interference is shifted from air to inside water. The fringe width then

Asked in GUJCET 2011 · Slabs, media and modified set-ups

Answer: (2) Decreases

Step-by-step solution

Given: the whole interference set-up is moved from air into water of refractive index n.

Idea: the fringe width is β=(λ D)/d, and inside a medium the wavelength becomes λ_w=λ/n.

The slit separation d and the screen distance D belong to the apparatus, so they do not change.

β_w=(λ D)/(nd)=β/n, and n>1 for water.

So the fringe width decreases, by the factor n (about 4/3 for water).

Why the other options are wrong

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