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

The film appears bright when the path difference (2μ t cosr) is equal to odd multiple of  λ2i.e. 2μtcosr=(2n−1) λ/2      where  n=1,  2,  3  .....∴   λ=4μ t cosr(2n−1)=4×1.4×10,000×10−10×cos0(2n−1)=56000(2n−1)Å∴  λ=56000 Å,18666 Å,8000 Å,6222 Å,5091 Å,4308 Å,3733 Å.            The wavelength which are not within specified range are to be refracted.