Answer:
Frequency
Explanation:
The property of waves that remains unchanged as it crosses the boundary of one medium to another is the frequency of the wave.
As a wave moves from one boundary to another, the wavelength and the speed of the wave changes.
The speed of the wave is product of wavelength and frequency. Also, the wavelength of the wave is function of the distance between successive crests or troughs of a wave.
The frequency of a wave is the number of waves that crosses a medium per unit of time.
