Question

Light is incident on a glass plate $(\mu =1.5)$ such that angle of refraction is ${60}^{\circ }.$ Dark band is observed corresponding to the wave length of $6000\mathring{A}$. If the thickness of glass plate is $1.2\times {10}^{-3}\text{mm}.$ Calculate the order of the interference band for reflected system .

• A. $4$
• B. $5$
• C. $3$
• D. $2$

Answer 1

The correct option is C $3$

Given, $\mu =1.5,r={60}^o,\lambda =6000\mathring{A}=6\times {10}^{-7}\text{m}$
$t=1.2\times {10}^{-3}\text{mm}=1.2\times {10}^{-6}\text{m}$
For dark band in the reflected light $2\mu t\cos r=n\lambda$
$n=\frac{2\mu t\cos r}{\lambda }=\frac{2\times 1.5\times 1.2\times {10}^{-6}\times \cos {60}^{\circ }}{6\times {10}^{-7}}$
$=\frac{2\times 1.5\times 1.2\times {10}^{-6}\times \frac{1}{2}}{6\times {10}^{-7}}=3$