As light move from air into water, it not only slows but the wavelength changes. the animation below illustrates how the wavelength become shorter in the denser medium of water... in the above equation, the frequencies cancel because frequency does not change as light moves from one medium to another.

Introduction:
When light travels from one medium to another, it undergoes a phenomenon called refraction. Refraction is the bending of light as it passes from one medium to another due to a change in its speed. While several properties of light change during refraction, such as its speed and direction, the frequency of light remains constant.

Explanation:
1. Frequency of light:
Frequency is a property of light that refers to the number of wave cycles that pass a given point in a second. It is measured in Hertz (Hz), which represents the number of cycles per second. The frequency of light determines its color, with different frequencies corresponding to different colors in the visible spectrum.

2. Refraction of light:
Refraction occurs when light passes through a boundary between two different mediums, such as air and water or air and glass. The change in speed of light as it enters a different medium causes it to bend. This bending is due to the change in the refractive index of the medium, which is a measure of how much the speed of light changes when passing through that medium.

3. Speed and wavelength change:
When light travels from one medium to another, its speed changes. This change in speed is accompanied by a change in the wavelength of the light. The wavelength determines the distance between successive wave peaks or troughs. As the speed of light changes, the wavelength adjusts to maintain a constant frequency.

4. Constant frequency:
According to the wave equation (c = λf), where c is the speed of light, λ is the wavelength, and f is the frequency, the product of wavelength and frequency remains constant for a given medium. As the speed and wavelength change during refraction, the frequency must remain constant to satisfy this equation. Therefore, when light travels from one medium to another, the frequency remains the same.

Conclusion:
The frequency of light does not change when it travels from one medium to another because the speed and wavelength of light adjust to maintain a constant frequency. This phenomenon is explained by the wave equation, where the product of wavelength and frequency remains constant in a given medium. Refraction may alter the speed and direction of light, but the frequency, which determines the color of light, remains unchanged.