Question

When light enters from air to water, then its frequency:
(A) increases and speed decreases
(B) is same but the wavelength is smaller in water than in air
(C) is same but the wavelength in water is greater than in air
(D) decreases and wavelength is smaller in water than in air

Answer 1

The frequency only deals with the number of times the considered light waves repeat per unit period of time. It depends only on the nature of the light and not on the medium. But the wavelength and the velocity depends on the medium.

Useful formula:
The formula of the velocity is given by

$v = n\lambda$

Where $v$ is the velocity of the light, $n$ is the frequency of the light and $\lambda$ is the wavelength of the light.

Complete step by step solution:
When the light moves from the medium of air to that of the medium of water, then due to the changing of medium, the wavelength changes. And due to the change of the wavelength, the speed of the light waves also changes due to the following relation.

$v = n\lambda$

But the frequency only depends on the nature of the source of the light and the light waves but it does not have any effect on the medium. So the change in the medium from air and the water, does not have any influence on the frequency of the light waves. Hence the frequency does not change when the propagating medium changes. The answer is the frequency is the same but the wavelength is small in water when compared to that of the air.

Thus the option (B) is correct.

Note: Remember that the wavelength of the light waves will be increased when it moves to the rarer medium and it decreases when it moves to the denser medium. In the above given condition, the air is the rarer medium and the water is the denser medium.