In a Young’s double‑slit experiment the slit separation is $d=0.25\text{ mm}$, the wavelength of light is $\lambda =600\text{ nm}$ and the screen is placed $L=1.8\text{ m}$ from the slits. (a) Find the fringe spacing $\beta$ between adjacent bright fringes on the screen. (b) What is the distance from the central maximum to the 12th bright fringe?

Step-by-step solution

1. Use the fringe spacing formula $\beta =\frac{\lambda L}{d}$ for Young’s double‑slit experiment.
2. Convert all quantities to metres: $d=0.25\text{ mm}=2.5\times 10^{-4}\text{ m}$, $\lambda =600\text{ nm}=6\times 10^{-7}\text{ m}$, $L=1.8\text{ m}$.
3. Insert into $\beta =\frac{6\times 10^{-7}\times 1.8}{2.5\times 10^{-4}}=4.32\times 10^{-3}\text{ m}=4.32\text{ mm}$.
4. For the 12th bright fringe, multiply the fringe spacing by the fringe number: $x_{12}=12,\beta =12\times 4.32\text{ mm}=51.84\text{ mm}$.

Concept tested: Fringe spacing in double‑slit experiment

Published 4 October 2026 · How this solution was prepared and checked