Question

A beam of light consisting of two wavelengths, 650 nm and 520 nm, is used to obtain interference fringes in a Young’s double-slit experiment.
(a) Find the distance of the third bright fringe on the screen from the central maximum for wavelength 650 nm.
(b) What is the least distance from the central maximum where the bright fringes due
to both the wavelengths coincide?

Answer 1

(a) Distance of the nth bright fringe on the screen from the central maximum is given by the relation,

b) Let the nth bright fringe due to wavelength λ₂ and (n − 1)^th bright fringe due to wavelength λ₁ coincide on the screen. We can equate the conditions for bright fringes as:​

Hence, the least distance from the central maximum can be obtained by the relation:

Note: The value of d and D are not given in the question.