Using a Michelson Interferometer and a Light Source of 400 $\times$

Multiple Choice

Using a Michelson interferometer and a light source of 400 nm,a change from bright to dark indicates a change in distance of

A) 2.0 $\times$ 10^-4 mm.
B) 4.0 $\times$ 10^-4 mm.
C) 3.0 $\times$ 10^-4 mm.
D) 1.0 $\times$ 10^-4 mm.