Figure illumstrates an interferometer used in measurements of refreactive indices of transparent substances. Here `S` is

a narrow slit illuminated by monochromatic light with wavelength `lambda = 589 nm`, 1 and 2 are identical tubes with air of length `l = 10.0 cm` each, `D` is a diaphragm with two slits. After the air in tube `I` was replaced with ammonia gas, the interference pattern on the screen `Sc` was displaced upwards by `N = 17` fringes. The refreactive indec of air is equal to `n = 1.000277`. Determine the refractive index of ammonia gas.

Explanation

No. of fringes shifted `= (n' - n) (l)/(lambda) = N`
so `n' = n + (N lambda)/(l) = 1.000377`.