(a) In Young.s double slit experiment using monochromatic light of wavelength `600` nm, interference pattern was obtained on a scren kept `1.5` m away from the plane of the two slits. Calculate the distance between the two slits. Fringe `"separation" // "fringe"` width was found to be `1.0` mm.
(b) When a fish is observed underwater at depth of `3` m appears to be at a depth of `2` m. What is the refractive index of water ?

Explanation

### Solution:

**(a)** To find the distance between the two slits in Young's double slit experiment, we can use the formula for fringe width (β):

\[ \beta = \frac{d \cdot \lambda}{D} \]

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