Q. In Young's double slit experiment, the slits are 2 mm apart and are illuminated by photons of two wavelengths λ₁ = 12000 Å and λ₂ = 10000 Å. At what minimum distance from the common central bright fringe on the screen 2 m from the slit will a bright fringe from one interference pattern coincide with a bright fringe from the other?
A) 8 mm
B) 6 mm
C) 4 mm
D) 3 mm
Explanation
[Solution from source] Fringe width β = λD/d: β₁ = (12000 × 10⁻¹⁰ × 2)/(2 × 10⁻³) = 1.2 mm and β₂ = 1.0 mm. Bright fringes coincide when n₁(1.2) = n₂(1.0), first at n₁ = 5, n₂ = 6, i.e. 6 mm from the centre. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.