Question

When a thin film of thickness $t$ is placed in the path of light wave emerging out of the slit, then increase in the length of optical path will be:

• A. $(\mu -1)t$
• B. $(\mu +1)t$
• C. $\mu t$
• D. $\frac{\mu }{t}$

Answer 1

The correct option is A $(\mu -1)t$

$\Delta x=s_{2}p-[s_{1}p+\mu t-t]$
$=s_{2}p-s_{1}p-(\mu -1)t$
$=\frac{yd}{D}-(\mu -1)t$

So, increase in length is $=\mu t-t$ $=(\mu -1)t$