A Michelson Interferometer Can Be Used to Measure Very Small $560 \mathrm{~nm}$

Multiple Choice

A Michelson interferometer can be used to measure very small distances by adjusting the length of one arm while keeping the other arm fixed. As the arm length is slowly changed, the central region of the screen changes from bright to dark and back to bright a number of times, and these bright fringes are counted. Suppose the interferometer uses light of wavelength $560 \mathrm{~nm}$ , and as one arm is lengthened by an amount $\Delta \mathrm{x}$ , exactly 79 bright fringes are counted. What is the distance $\Delta x$ ?

A) $44.2 \mu \mathrm{m}$
B) $22.1 \mu \mathrm{m}$
C) $7.09 \mathrm{~nm}$
D) $22.4 \mu \mathrm{m}$
E) $44.8 \mu \mathrm{m}$
F) $11.1 \mu \mathrm{m}$