Question

A monochromatic beam of light falls on YDSE apparatus at some angle $(\text{say}\theta )$ as at some angle $(\text{say}\theta )$ 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 on angle $\theta$, thickness of plate $t$, and refractive index of glass $\mu$

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