Interference fringes are produced by a double slit arrangement
and a piece of plane parallel glass of refractive index 1.5 is interposed in one of
the interfering beam. If the fringes are displaced through 30 fringe widths for
light of wavelength `6xx 10^(-5) cm`, find the thickness of the plate.

Explanation

Path difference due to the glass slab,`Delta x = (mu - 1) t`. Thirty fringe are displaced due to the slab.
Hence, `Delta x = 30 lambda implies (mu - 1) t = 30 lambda`
or `t = (30 lambda)/(mu - 1) = (30 xx 6 xx 10^(-5))/(1.5 - 1) = 3.6 xx 10^(-3) cm`