Bright and Dark Fringes Are Seen on a Screen When $d = \frac { \lambda _ { \text {aiy } } } { 2 }$

Multiple Choice

Bright and dark fringes are seen on a screen when light from a single source reaches two narrow slits a short distance apart.The locations of bright and dark fringes can be interchanged if a thin film is placed in front of one of the slits.The minimum thickness of this film must be

A) $d = \frac { \lambda _ { \text {aiy } } } { 2 }$ .
B) $d = \frac { \lambda _ { \text {oir } } } { 2 n _ { \text {film } } }$ .
C) $d = \frac { \lambda _ { \text {oir } } } { \left( n _ { film } - 1 \right) }$ .
D) $d = \frac { \lambda _ { \text {air } } } { 2 \left( n _ { film } - 1 \right) }$ .
E) $d = \frac { \lambda _ { \text {air } } } { n _ { \text {film } } }$ .