If two media have identical impedances, what happens to reflections?

If two media have identical impedances, what happens to reflections?

• A. The reflected wave is eliminated.
• B. No reflection occurs.
• C. Refraction occurs with no change in speed.
• D. The frequency doubles.

Answer: (B)

When an ultrasound wave encounters an interface, reflections come from impedance differences between the two media. The acoustic impedance is Z = ρc, and at normal incidence the reflection is zero when Z1 equals Z2, because the boundary then presents no impedance contrast. In that case all the incident energy is transmitted into the second medium, so no reflected wave exists. The wave can still refract if the speeds differ (following Snell’s law), and its speed and wavelength may change, but the frequency remains unchanged across the boundary. That’s why the correct statement is that no reflection occurs. The other ideas contradict how impedance mismatch, refraction, and frequency conservation work.

When an ultrasound wave encounters an interface, reflections come from impedance differences between the two media. The acoustic impedance is Z = ρc, and at normal incidence the reflection is zero when Z1 equals Z2, because the boundary then presents no impedance contrast. In that case all the incident energy is transmitted into the second medium, so no reflected wave exists. The wave can still refract if the speeds differ (following Snell’s law), and its speed and wavelength may change, but the frequency remains unchanged across the boundary. That’s why the correct statement is that no reflection occurs. The other ideas contradict how impedance mismatch, refraction, and frequency conservation work.