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

4308 Å, 5091 Å, 6222 Å

The film appears bright when the path difference $(2μt\, \cos r)$ is equal to odd multiple of λ/2,

i.e., $2μt\, \cos r = (2n - 1) \frac{λ}{2}$

where n = 1, 2, 3 . . . . .

$∴ λ =\frac{4μt\, \cos r}{(2n -1)}$

$=\frac{4×1.4×10000×10^{-10}×\cos 0}{(2n-1)}=\frac{56000}{(2n-1)}Å$

$∴ λ = 56000 Å, 1866 Å, 11200 Å, 8000 Å, 6222 Å, 5091 Å, 4308 Å, 3733 Å$

The wavelengths, which are not within specified range, are to be neglected.