Answer:
2 x 10ā»ā· m
Explanation:
Since the light from two slits interfere to form dark fringe on the screen ,
the path difference must be odd multiple of Ī» / 2 .
Since it is the first dark fringe the factor of odd multiple must be one.
So path difference must be
( 2n +1 ) x Ī» / 2 .
for first dark fringe
n = 0
path difference Ā = Ā Ī» / 2
400 x 10ā»ā¹ / 2 m
= 2 x 10ā»ā· m