Question

A monochromatic beam of light is used for the formation of fringes on the screen by illuminating the two slits in the Young's double slit interference experiment. When a thin film of mica is interposed in the path of one of the interfering beams,

• A. the fringe width increases
• B. the fringe width decreases
• C. the fringe width remains the same but the pattern shifts
• D. the fringe pattern disappears

Answer 1

The correct option is C the fringe width remains the same but the pattern shifts

If a thin film of mica is placed in front of slit $S_1$ then the pattern is shifted upwards and if the thin mica sheet is place in front of $S_2$ then the pattern will shit downwards.
There will be no change in the fringe-width as there is no change in the distance $D$ between the slits and the screen; no change in the distance $d$ between the slits and the wavelength $\lambda$.