White light may be considered to be a mixture of wave with λ ranging between 3900A0 and 7800A0  . An oil film of thickness 10,000A0 is examined normally by reflected light. If μ=1.4 , then the film appears bright for

The film appears bright when the path difference 2μtcosr is equal to odd multiple of λ/2.i.e., 2μtcosr=2n−1λ2 where  n = 1,2,3… ∴λ = 4μtcosr2n−1=4×1.4×10,000×10−10×cos02n−1=560002n−1A0   For normal incidence r= 0o ⇒λ=56000A0,18666A0,11200A0, 8000A0,6222A0,5091A0,4308A0,3733A0.   The wavelengths which are not within specified range are to be refracted.