Chapter 16, Problem 1CQ

To determine

Explain the reason behind the cause of reflection at the surface of lake and different speed of light in air and water.

Answer to Problem 1CQ

The speed of light in water is less than the speed of light in air. A part of light is reflected at the water – air interface.

Explanation of Solution

Write the expression for speed of light in another medium.

  $v=\frac{c}{\mu }$        (I)

Here, $v$ is the velocity of light in the medium, $c$ is the speed of light in air and $\mu$ is the refractive index of the medium.

When the light falls at the surface of lake, it gets partially reflected and partially refracted because the surface of a lake is partially shiny.

The light, when incident on top surface of lake, gets reflected back in the air due to which the reflected light has same velocity as that of the incident light because the light is reflected back in same medium, same refractive index.

When the light falls on the surface of the lake, it gets refracted and the speed of light in water is not same as that of the speed of light.

 The refractive index of water is greater than the refractive index of air. From equation (I), higher the refractive index lower will be the speed of light in that medium. Therefore, the speed of light in water is less than the speed of light in air.

Conclusion:

Thus, the speed of light in water is less than the speed of light in air.

A part of light is reflected at the water – air interface.