Chapter 27, Problem 13A

Does the time delay between the absorption and reemission of light affect the average speed of light in a material? Explain.

To determine

Whether time delay can affect the average speed of light or not.

Explanation of Solution

Introduction: Whenever light falls on the surface of material, the following phenomena can occur:

1. Light gets reflected form the surface or,
2. Transmitted from the material or,
3. Some part it gets reflected and some part gets transmitted.

We know that whenever light fall and passes through a material it’s speed get slow down i.e. travelling from rarer medium to denser medium. During the transmission light particle also called photon absorbed by the atom of a material due to which electro of the atom make a transition from lower energy state to higher energy state. After short interval, it will fall back to lower excited state with the emission of same photon and this phenomenon continuous till photon comes out of the material. Hence, this time delay in transition from one excited state to another excited state will slow down the average speed of the light in a material from the actual speed of light $3\times {10}^8{\text{ms}}^{-1}$ .

Conclusion: The average speed of light passing through a material decreases due to time delay in absorption and reemission of a light.