In a Young’s double‑slit experiment the slit separation is $d=0.20\text{mm}$ and monochromatic light of wavelength $\lambda =550\text{nm}$ is used. The screen is placed $L=1.00\text{m}$ from the slits. Calculate the fringe spacing $\beta$ (distance between two consecutive bright fringes) on the screen.

Step-by-step solution

1. Convert all quantities to metres: $d=0.20\times 10^{-3}\text{m}$, $\lambda =550\times 10^{-9}\text{m}$, $L=1.00\text{m}$.
2. Use the fringe spacing formula for small angles: $\beta =\frac{\lambda L}{d}$.
3. Substitute the values: $\beta =\frac{550\times 10^{-9}\times 1.00}{0.20\times 10^{-3}}$.
4. Calculate the numerator: $550\times 10^{-9}=5.5\times 10^{-7}$.
5. Divide: $\frac{5.5\times 10^{-7}}{0.20\times 10^{-3}}=\frac{5.5\times 10^{-7}}{2\times 10^{-4}}=2.75\times 10^{-3}\text{m}$.
6. Convert metres to millimetres: $2.75\times 10^{-3}\text{m}=2.75\text{mm}$.

Concept tested: Young's double‑slit fringe spacing

Published 7 September 2026 · How this solution was prepared and checked