question_answer

A double slit, \[{{S}_{1}}-{{S}_{2}}\]is illuminated by a light source S emitting light of wavelength\[\lambda \]. The slits are separated by a distance d. A plane mirror is placed at a distance D in front of the slits and a screen is placed at a distance 2D behind the slits. The screen receives light reflected only by the plane mirror. The fringe-width of the interference pattern on the screen is       

A) \[\frac{D\lambda }{d}\]

B) \[\frac{2D\lambda }{d}\]

C) \[\frac{3D\lambda }{d}\]

D) \[\frac{4D\lambda }{d}\]