A beam of light is incident vertically on a glass slab of thickness \(1~\text{cm},\) and refractive index \(1.5.\) A fraction \(A\) is reflected from the front surface while another fraction \(B\) enters the slab and emerges after reflection from the back surface. The time delay between them is:
1. | \(10^{-10}~\text{s}\) | 2. | \(5\times 10^{-10}~\text{s}\) |
3. | \(10^{-11}~\text{s}\) | 4. | \(5\times 10^{-11}~\text{s}\) |