On a hot summer night, the refractive index of air is smallest near the ground and increases with height from the ground. When a light beam is directed horizontally, the Huygens' principle leads us to conclude that as it travels, the light beam:

Two light waves having the same wavelength \(\lambda\) in vacuum are in phase initially. Then the first wave travels a path of length \(L_1\) through a medium of refractive index \(n_1\) while the second wave travels a path of length \(L_2\) through a medium of refractive index \(n_2\). The two waves are then combined to observe interference. The phase difference between the two waves, now, is:
1. \( \frac{2 \pi}{\lambda}\left(n_2 L_1-n_1 L_2\right)\)
2. \( \frac{2 \pi}{\lambda}\left(\frac{L_1}{n_1}-\frac{L_2}{n_2}\right) \)
3. \(\frac{2 \pi}{\lambda}\left(n_1 L_1-n_2 L_2\right) \)
4. \(\frac{2 \pi}{\lambda}\left(\frac{L_2}{n_1}-\frac{L_1}{n_2}\right) \)