What happens to ultrasonic waves when they enter a denser material from a less dense material?

When ultrasonic waves transition from a less dense material to a denser material, the waves actually slow down. This behavior is due to the density and elastic properties of the materials involved. In general, the speed of sound in a medium is influenced by its density and elasticity; sound travels slower in denser materials because the particles are more tightly packed, which requires more energy for the wave to propagate through.

Therefore, while moving from a less dense to a denser medium, the density increase and the associated propagation characteristics lead to a decrease in wave speed. This phenomenon is fundamentally rooted in the principles of acoustics and material science that govern wave behavior in different media.

In contrast, the other options do not accurately reflect the behavior of ultrasonic waves in this scenario. For instance, waves do not reflect fully in this instance; instead, partial reflection may occur at the interface, depending on the acoustic impedance mismatch. Disappearance of waves is not a possibility, as waves cannot simply vanish upon entering another medium.