Question

A monochromatic beam of light falls on $YDSE$ apparatus at some angle (say $\theta$) as shown in figure. A thin sheet of glass is inserted in front of the lower slit $s_{2}$. The central bright fringe (path difference = 0) will be obtained

• A. at $O$
• B. above $O$
• C. below $O$
• D. anywhere depending on angle $\theta$ thickness of plate $t$, and refractive index of glass $\mu$

Answer 1

Answer: (D)

If $d$ sin $\theta = \left(\mu - 1 \right)t$, central fringe is obtained at $O$.
If $d$ sin $\theta > \left(\mu - 1 \right)t$, central fringe is obtained above $O$.
If $d$ sin $\theta < \left(\mu - 1 \right)t$, central fringe is obtained below $O$.