Question

the visible light of wavelength 350 NM to 800 NM is incident normally on a thin film with refractive index 4 it is observed that three wavelength 720 NM, 480 NM and 360 NM are compulsory missing from reflected spectra find the minimum thickness of a film.

Answer 1

As per the given question,

Refractive index of the film$(\mu) = 4$

Minimum wavelength $(\lambda) = 360nm$

$2\mu t = (2n-1)\lambda$

Now, substituting the values,

$t= \frac{(2n-1)\lambda}{2\mu}$

$\Rightarrow t= \frac{(2-1)\times 360\times 10^{-9}}{2\times 4}$

$\Rightarrow t = 45\times 10^{-9}m$