For the wavelength which will not be seen

\(2 \mu t \cos r=n \lambda\)

\(r= 0 \text { (Normal incidence) }\)

\(2(1.4) 10^{-4} \times 10^{-2} \cos 0^{\circ}=n \lambda\)

\(n \lambda= 2.8 \times 10^{-6}\)

\(n \lambda=28000 \times 10^{-10}\)

\(\lambda=\frac{28000}{n} \,A\)

For possible wavelength \(n=1,2,3, \ldots \ldots\)