What Minimum Thickness of Oil $\mathrm{n}=1.50)$ On the Surface of Water

Multiple Choice

What minimum thickness of oil ( $\mathrm{n}=1.50)$ on the surface of water $(\mathrm{n}=1.33)$ would give constructive interference for $550 \mathrm{~nm}$ light at normal incidence?

A) $92.0 \mathrm{~nm}$
B) $183 \mathrm{~nm}$
C) $80.0 \mathrm{~nm}$
D) $275 \mathrm{~nm}$
E) $78.0 \mathrm{~nm}$