Reflection magnitude at a boundary depends on which factor?

• A. The difference in impedances of the two media at the boundary
• B. The sum of the impedances
• C. The product of impedances
• D. The density difference only

Answer: (A)

The amount of reflection at a boundary is governed by how different the impedances of the two media are. Impedance is what the wave has to push against as it moves into the next medium, and the boundary conditions require a certain balance of pressure (or E-field) and velocity (or H-field) across the boundary. When you work through the boundary conditions for a wave incident normally, you arrive at a reflection coefficient that depends on the difference between the two impedances relative to their sum: larger mismatch means more of the wave is reflected, while equal impedances give no reflection.

So, the key factor is the impedance difference between the two media. The sum or product of impedances don’t determine the reflected fraction, and density difference alone isn’t enough because impedance also depends on wave speed (Z = ρc in acoustics, for example). A greater mismatch in Z leads to a stronger reflection, and matching impedances yields minimal or no reflection.