A surface of a glass plate is covered with a thin layer of water. A light with wavelength = 0.680 µm incident at an angle 30º. Due to evaporation of the water layer, the intensity of the reflected light change periodically, time interval between the appearances of maximum intensity is equal to 15.0 min. Find the rate of decrease of the water layer thickness in µm/hr (µ g = 1.50, µ w = 1.34)
Text Solution
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Ans. 1
Sol.
Condition for constructive interference
2 µt cos r = n λ
sin 30º = 1.34 sin r
sin r = 
or 
cos r = 
2 µt cos r = n λ …… (1)
2µ (t – Δ t) cos r = (n – 1) λ …….(2)
equation (1) – equation (2)
2µ Δ t cos r = λ
Δ t = 
Rate of decrease = 
= 1.01 µm/hr 
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