Using a Michelson Interferometer and a Light Source of 400

Multiple Choice

Using a Michelson interferometer and a light source of 400 nm, a change from bright to dark indicates a change in the difference of arm lengths of

A) 2.0 × 10^-4 mm.
B) 4.0 × 10^-4 mm.
C) 3.0 × 10^-4 mm.
D) 1.0 × 10^-4 mm.