Walker, Halliday & Resnick — Interference: Problem 61

61 In Fig. 35-4, assume that the two light waves, of wavelength $515 \text{ nm}$ in air, are initially out of phase by $\pi \text{ rad}$. The indexes of refraction of the media are $n_1 = 1.45$ and $n_2 = 1.75$. What are the (a) smallest and (b) second smallest value of $L$ that will put the waves exactly in phase once they pass through the two media?