Question

In YDSE, find the thickness of a glass slab $(\mu=1.5)$ which should be placed before the upper slit $S_{1}$ so that the central maximum now lies at a point where $5$ th bright fringe was lying earlier (before inserting the slab). Wavelength of light used is $5000 \,\mathring{A}$.

• A. $5 \times 10^{-6} m$
• B. $3 \times 10^{-6} m$
• C. $10 \times 10^{-6} m$
• D. $5 \times 10^{-5} m$

Answer 1

Answer: (A)

According to the question,
shift $=5$ fringe widths
$\Rightarrow \frac{(\mu-1) t D}{d}=\frac{5 \lambda D}{d}$
$\therefore t=\frac{5 \lambda}{\mu-1}=\frac{25000}{1.5-1}=50,000 \,\mathring{A}$
$=5 \times 10^{-6} m$