A Level Physics Practice Exam

If a reflected ultrasound wave returns with no significant loss in amplitude, what happens to the particles in the medium?

1. All particles move in the same direction

2. Particles oscillate in phase with fixed nodes

3. Particles move randomly without correlation

4. Particles stop moving entirely

The correct answer is: Particles oscillate in phase with fixed nodes

When a reflected ultrasound wave returns with no significant loss in amplitude, the interaction of the wave with the medium causes the particles within that medium to oscillate. This oscillation results from the compressions and rarefactions created by the ultrasound wave as it travels through. In this scenario, since the amplitude remains significant, the energy of the wave is effectively transmitted through the medium, allowing the particles to oscillate in a manner that is both periodic and coherent. This means the particles in the medium oscillate in phase, meaning they reach their maximum displacement at the same time and return to their equilibrium position together. Additionally, the presence of fixed nodes refers to points along the medium where there is no movement of particles—these nodes occur in standing waves, which can happen due to the interference of the incident and reflected waves. Thus, the correct choice highlights this coherent oscillation of particles in phase, emphasizing their organized and predictable motion rather than random or static behavior.