"A light wave passes from air into water. Which of the following is true? 1. Its frequency remains unchanged, but its wavelength decreases. 2. Its wavelength remains unchanged, but its frequency decreases. 3. Its wavelength remains unchanged, but its frequency increases. 4. None of these 5. Both its wavelength and its frequency remain unchanged."

1. Its frequency remains unchanged, but its wavelength decreases.

Explanation:

When the light wave passes from air to water then the speed of light wave reduces due to water being optically denser than the air.

According to the conservation of energy the energy of the photons before and after the refraction remains same and as the energy of the wave is dependent on the frequency so the frequency of the wave does not changes.

We know the relation between the frequency, speed and the wavelength of the light as:

where:

frequency of the wave

speed of light in the medium

wavelength of the wave

Now from the above relation we can observe that if frequency has to remain constant while the speed of the wavelength reduces in and optically denser medium then the wavelength must also reduce proportionally.

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