Which two quantities describe how much energy is reflected and transmitted at an interface?

• A. Impedance and Attenuation
• B. Wavelength and Frequency
• C. Intensity Reflection Coefficient and Intensity Transmission Coefficient
• D. Pressure and Phase

Answer: (C)

When a wave reaches an interface, part of its energy bounces back into the first medium and part passes into the second. The quantities that describe how much energy goes into each part are the intensity-based coefficients: the intensity reflection coefficient and the intensity transmission coefficient. They tell you the fractions of the incident energy that are reflected and transmitted, respectively. In a lossless boundary, these fractions add up to one, with the exact values depending on the impedance mismatch between the media. For normal incidence, the reflected-energy fraction is given by ((Z2 − Z1)/(Z2 + Z1))^2, and the transmitted-energy fraction is the remainder (or equivalently 1 minus the reflection fraction). Impedance and attenuation relate to how waves propagate and lose energy, but they aren’t the fractions of energy reflected or transmitted. Wavelength and frequency describe wave properties, not energy partition at an interface. Pressure and phase are field quantities, not the specific energy-carrying fractions.