A plane wave of monochromatic light falls normally on a uniform thin film of oil which covers glass plate. The wavelength of source can be varied continuously. Complete destructive interference is observed for λ=5000A0 and λ=7000A0 and for no other wavelength in between. If μ of oil 1.3 and that of glass is 1.5, the thickness of the film will be
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a
6.738×10−5cm
b
5.7×10−5cm
c
4×10−5cm
d
2.8×10−5cm
answer is A.
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Detailed Solution
In this case, both the rays suffer a phase change of 1800 and the condition for destructive interference is 2nd=(m+12)λ1 2nd=(m+32)λ2 ∴m+12m+32=λ2λ1=50007000=57 And d=(m+12)λ12n=2.5×70002×1.3 =6738A0= 6.738×10−5cm
A plane wave of monochromatic light falls normally on a uniform thin film of oil which covers glass plate. The wavelength of source can be varied continuously. Complete destructive interference is observed for λ=5000A0 and λ=7000A0 and for no other wavelength in between. If μ of oil 1.3 and that of glass is 1.5, the thickness of the film will be