I get that when a wave enters a more dense medium, the frequency does not change - only wavelength and velocity. But why does the amplitude decrease? What does this mean?
When a wave enters a more dense medium, such as going from air to water, the amplitude of the wave decreases. This is because the wave encounters resistance or an impediment due to the denser medium.
To understand why the amplitude decreases, let's consider a basic example of a water wave moving from deep water to shallow water. In deep water, the wave encounters fewer obstacles and has more space to propagate, allowing it to have a larger amplitude. However, as the wave approaches shallow water, it encounters the seabed, which creates resistance to the wave motion.
The resistance forces the wave to adjust. As a result, some of the wave energy is transferred to the seabed, causing the amplitude to decrease. This transfer of energy can also be explained by the conservation of energy principle. Since the total energy of the wave is constant, when the wave encounters resistance, some of its energy is dissipated, resulting in a decrease in amplitude.
The decrease in amplitude means that the wave becomes less intense or powerful as it moves into the more dense medium. In the context of water waves, this can be observed as the wave becoming smaller and less pronounced. In other words, the wave loses some of its "strength" or "height" as it enters the more dense medium.
To sum it up, the decrease in amplitude of a wave when it enters a more dense medium is due to the resistance or impediment encountered by the wave, which causes the wave to transfer energy and become less intense or powerful.