Question

White light is incident normally on a soap film of thickness $15\times {10}^{-5}cm$ and refractive index $1.33$. Which wavelength is reflected maximum in the visible region?

• A. $26000A^o$
• B. $8866A^o$
• C. $5320A^o$
• D. $3800A^o$

Answer 1

The correct option is C $5320A^o$

$n_1=n_3<n_2$

A phase change of $\pi$ takes place at point A, but not at B. Hence for a constructive interference to be obtained, the condition is-

$2\mu d\cos r=(m-\frac{1}{2})\lambda$

Since incidence is normal, $\cos r=1$

$\lambda =\frac{2\mu d}{(m-\frac{1}{2})}$

$\lambda$ lies in visible region only for m=8.

${\lambda }_{m=8}=5320A^{\circ }$