Q 12-10-133JEE MainJEE Main 2019 (9 Jan, Shift 2)Medium
In a Young's double slit experiment, the slits are placed $0.320\ \text{mm}$ apart. Light of wavelength $\lambda = 500\ \text{nm}$ is incident on the slits. The total number of bright fringes that are observed in the angular range $-30^\circ \le \theta \le 30^\circ$ is:
Answer: (B) $641$
Bright fringes satisfy $d\sin\theta = n\lambda$. At $\theta = 30^\circ$:
$$n = \frac{d\sin30^\circ}{\lambda} = \frac{0.32\times10^{-3}\times0.5}{500\times10^{-9}} = 320$$
So $n$ runs from $-320$ to $+320$, giving $2(320)+1 = 641$ bright fringes.
Solution by Sreeraj P, M.Sc Physics