A beam of light consisting of wavelengths 650 nm and 550 nm illuminates the Young's double slits with separation of 2 mm such that the interference fringes are formed on a screen, placed at a distance of 1.2 m from the slits. The least distance of a point from the central maximum, where the bright fringes due to both the wavelengths coincide, is 10-5 m.
Answer
429
Solution
Bright fringes coincide when n1 1 = n2 2
n1 650 = n2 550 n1n2 = 1113
Least distance: n1 = 11 (for 650 nm)
y = n1 1 Dd = 11 650 10-9 1.22 10-3
y = 4.29 10-3 m = 429 10-5 m
Answer: 429