In Young's double slit experiment, the slits are 2 mm apart and are illuminated by photons of two wavelengths 1 = 12000 A and 2 = 10000 A. 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 ?
- 4 mm
- 3 m
- 8 mm
- 6 mm
Answer
(4) 6 mm
Solution
For coincidence of bright fringes: n1 1 = n2 2
⇒ n1n2 = 2 1 = 1000012000 = 56
Minimum: n1 = 5 (for 1), n2 = 6 (for 2)
y = n1 1 Dd
= 5 12000 10-10 22 10-3
= 6 10-3 m
∴ y = 6 mm
Answer: (4)