Why frequency remains unchanged during reflection​

Although the wave slows down, its frequency remains the same, due to the fact that its wavelength is shorter. When waves travel from one medium to another the frequency never changes. As waves travel into the denser medium, they slow down and wavelength decreases.

When waves travel from one medium to another the frequency never changes.

Explanation:

As waves travel into the denser medium, they slow down and wavelength decreases. Part of the wave travels faster for longer causing the wave to turn. The wave is slower but the wavelength is shorter meaning frequency remains the same.

this is poiselle's equation from fluid mechanic in physics. mech eng