Question

The distance of a point on the screen from two slits in biprism experiment is $1.8\times {10}^{-5}m$ and $1.23\times {10}^{-5}m$. If wavelength of light used is $6000\mathring{A}$, the fringe formed at that point is

• A. ${10}^{th}$ bright
• B. ${10}^{th}$ dark
• C. $9^{th}$ bright
• D. $9^{th}$ dark

Answer 1

The correct option is D $9^{th}$ dark

The correct answer is option(D).

Two images of slit acts as two new slits,

so, the path difference between light travelling to the point from the two sources,

$\Delta x=(1.8-1.23)\times {10}^{-5}m$

Equating the path difference with multiple wavelength ,

$\Rightarrow \Delta x=n\lambda$

$\Rightarrow 0.57\times {10}^{-5}=n\times 6000\times {10}^{-10}$

$n=\frac{19}{2}=9+\frac{1}{2}$

This is the condition of dark fringe.

So answer is 9${}^{th}$ dark fringe.