Q 12-10-035JEE MainJEE Main 2026 (4 Apr, Shift 2)Medium
In a double slit experiment, when one of the slits is covered by a transparent mica sheet of refractive index $1.56$, the central fringe shifts to the position of $7^{\text{th}}$ bright fringe, obtained with both slits uncovered. If the light source wavelength is $450$ nm, the thickness of mica sheet is $\alpha \times 10^{-9}$ m. The value of $\alpha$ is ______.
Numerical value type. Enter your answer.
Answer: 5625
Concept. A sheet adds optical path $(\mu-1)t$; the centre shifts by $n$ fringes when $(\mu-1)t=n\lambda$.
Step 1. Thickness. $t=\dfrac{7\times450}{0.56}=5625$ nm.
Answer. $\alpha=5625$.
Solution by Sreeraj P, M.Sc Physics
