Question

In YDSE apparatus shown in figure wavelength of light used is $\lambda$. The screen is moved away from the source with a constant speed $v$. Initial distance between screen and plane of slits was $D$ and at point $P$ on screen, $n^{th}$ order bright fringe is observed.

As the screen moves away value of $n$ will

• A. increase
• B. decrease
• C. remain constant
• D. first increase then decrease

Answer 1

The correct option is B decrease

The distance of $n^{th}$ order bright fringe from central maxima is
$y=\frac{n\lambda D}{d}$
or $n=\frac{yd}{\lambda D}$
As the screen moves away, $D$ increases and $n$ will decrease.