Question

two coherent monochromatic sets of wave will interfere constructively in the region of superposition only if the path difference between them is ?
(a) half wavelength
(b) quarter wavelength
(c) integral number of wavelength
(d) odd intergral number of wavelength

Answer 1

i think its integral number of wavelength

Answer 2

two coherent monochromatic sets of wave will interfere constructively in the region of superposition only if the path difference between them is integral multiple of wavelength of monochromatic light.

here you noticed that interference of two coherent waves constructively so, bright fringe will be form. in this case we just choose path difference = integral multiple of wavelength .

e.g., if wavelength of monochromatic light is $\lambda$ then, path difference is $\Delta{x}=n\lambda$
where n is integer.

hence, option (c) is correct.