Consider a ray of light incident from the air onto a slab of glass (refractive index n) of width d, at an angle $\theta$. The phase difference between the ray reflected by the top surface of the glass and the bottom surface is:

1. \(\frac{4 \pi d}{\lambda}\left(1-\frac{1}{n^2} \sin ^2 \theta\right)^{1 / 2}+\pi\)

2. \(\frac{4 \pi d}{\lambda}\left(1-\frac{1}{n^2} \sin ^2 \theta\right)^{1 / 2}\)

3. \(\frac{4 \pi d}{\lambda}\left(1-\frac{1}{n^2} \sin ^2 \theta\right)^{1 / 2}+\frac{\pi}{2}\)

4. \(\frac{4 \pi d}{\lambda}\left(1-\frac{1}{n^2} \sin ^2 \theta\right)^{1 / 2}+2\pi\)