Q. A monochromatic light passes through a glass slab (μ = 1.5) of thickness 9 cm in time t_{1}. If it takes a time t_{2} to travel the same distance through water (μ = 4/3). The value of (t_{1} − t_{2}) is

(a) 5 × 10^{-11} sec

(b) 5 × 10^{-8} sec

(c) 2.5 × 10^{-10} sec

(d) 5 × 10^{-10} sec

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Ans: (a)

Sol: Distance d= 9 × 10

^{−2}m , Speed in medium v= c/μ andtime t= distance/ speed

$ \displaystyle t_1 = \frac{9 \times 10^{-2}}{c/1.5} $

$ \displaystyle t_2 = \frac{9 \times 10^{-2}}{c/(4/3)} $

t_{1} – t_{2} = 5 x 10^{-11} sec