Q: A thin sheet of glass (μ = 1.5 ) of thickness 6 micron introduced in the path of one of the interfering beams in a double slit experiment . Shifts the central fringe to a position previously occupied by fifth bright fringe. Then he wavelength of light used is

(a) 6000 Å

(b) 3000 Å

(c) 4500 Å

(d) 7500 Å

Ans: (a)

Sol: $ \displaystyle (\mu – 1)t = n \lambda $

$ \displaystyle \lambda = \frac{(\mu – 1) t}{n} $

$ \displaystyle \lambda = \frac{(1.5 – 1) 6 \times 10^{-6}}{5} $

λ = 6000 Å