Here's why:
* Frequency is a property of the light source: The frequency of light is determined by the vibrations of the source that emitted it. Whether the light is traveling through air, water, or any other transparent medium, the source's vibrations don't change.
* Speed and wavelength change: What does change is the speed of light. Light travels slower in water than in air. This change in speed leads to a change in wavelength. Since the speed of light (c) is related to frequency (f) and wavelength (λ) by the equation c = fλ, the wavelength must decrease to compensate for the decreased speed.
In summary:
* Frequency (f) remains constant.
* Speed (c) decreases.
* Wavelength (λ) decreases.
