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 the same but the wavelength in water is greater than in air.
(D) decreases and wavelength is smaller in water than in air.

Answer 1

Hint : In order to solve this question we need to understand that when the ray of the light enters from one medium to the other medium, some parameters will become the same and some parameters will change. If the ray of light is entering from air into water some parameters of the light will be the same and some are different. Frequency is the number of waves that pass through a single given point in a specified amount of time.

Complete Step By Step Answer:
The frequency of light remains constant when the light ray is incident on the interface of two media. But light moves with different speeds in different media.
$\lambda f = c$
As a result, the wavelength $\lambda$ of light also changes at the interface. This, in turn, causes a change in direction, which is refraction. Clearly during refraction,
The frequency of light remains constant.
Speed of light and wavelength changes causing the incident light to deviate.
Speed and wavelength decrease when light travels from rare to denser medium.
When light enters from air to water, since energy remains the same, its frequency also remains the same. But the speed of light decreases, decreasing its wavelength. Speed of light can be given by
$\lambda f = c$
Were,
$\lambda {\text{ is the wavelength}}$
${\text{f is the frequency}}$
${\text{c is the speed of light}}$
$\Rightarrow f = \dfrac{c}{\lambda }$
$\Rightarrow \lambda = \dfrac{c}{f}$
Hence, when light enters from air to water, then its frequency is the same but the wavelength is smaller in water than in air.
So, the correct option is (B).

Note :
It should be remembered that the frequency of light remains constant. Speed and wavelength decrease when light travels from rarer to denser medium. Frequency and wavelength are inversely proportional, which means longer the wavelength, lower the frequency. All forms of light waves move through the vacuum at the same speed of 3 × 10⁸ meters per second. Frequency will be higher when the wavelength is short, as more crests or troughs pass through a specific point when the wavelength is short.