Which characteristic of waves does not change when they enter a different medium?

The frequency of a wave remains constant when it enters a different medium. This is because frequency is determined by the source of the wave and is unaffected by changes in the medium through which the wave travels. When a wave transitions from one medium to another, it experiences changes in speed and wavelength due to the differing properties of the new medium (such as density and elasticity), but the frequency stays the same.

Maintaining a constant frequency ensures that the wave's oscillations continue at the same rate, even though other characteristics may be altered. This principle is fundamental to understanding wave behavior as it transitions through various environments, making frequency a critical concept in wave physics.