Question

A thin plastic sheet of refractive index $1.6$ is used to cover one of the slits of a double slit arrangement. The central point on the screen is now occupied by what would have been the 7th bright fringe before the plastic was used. If the wavelength of light is $600$ nm, what is the thickness $\left(in\mu \right)$ of the plastic sheet?

• A. $7$
• B. $4$
• C. $8$
• D. $6$

Answer 1

Answer: (A)

$7$

The change in path length $=(\mu -1)t$.
This path length change the position of bright fringe upto seventh bright fringe.

So, $(\mu -1)t=n\lambda \Rightarrow t=\frac{n\lambda }{\mu -1}=\frac{7\times 600\times {10}^{-9}}{1.6-1}m=7\times {10}^{-6}m=7\mu m$