Question

In Young's experiment, the wavelength of red light is $7.8 \times 10^{-8} cm$ and that of blue light is $5.2 \times 10^{-8} cm$. Value of $n$ for which $(n+1)^{\text {th }}$ blue bright fringe coincides with $n^{\text {th }}$ red fringe is

• A. $1$
• B. $2$
• C. $3$
• D. $4$

Answer 1

Answer: (B)

As we know that $n_{1} \, \lambda _{1}=n_{2} \, \lambda _{2}$
$\therefore \frac{n_{2}}{n_{1}}=\frac{\lambda _{1}}{\lambda _{2}}=\frac{7.8 \times 10^{- 8}}{5.2 \times 10^{- 8}}=\frac{3}{2}$
Also $n_{2}-n_{1}=1$
$\therefore n_{1}=2$