At the boundary between 2 media the principle of ______ applies.

At the boundary between 2 media the principle of ______ applies.

• A. Conservation of momentum
• B. Reciprocity of energy
• C. Continuity of impedance
• D. Conservation of energy

Answer: (D)

When a wave reaches the boundary between two media, energy must be conserved. The energy carried by the incoming wave encounters the interface and is split into two parts: some reflects back into the first medium, and some transmits into the second. In an ideal, lossless boundary, the total energy flux leaving the boundary equals the energy flux arriving—the sum of the reflected and transmitted energy equals the incident energy. This is why conservation of energy is the principle at play for boundary behavior: it governs how energy is partitioned between reflection and transmission. Impedance continuity isn’t the fundamental rule here; impedance can differ on the two sides, and the mismatch determines how much energy is reflected versus transmitted. The boundary conditions that actually govern the fields require continuity of the tangential electric and magnetic fields, which leads to those reflection and transmission effects, but without claiming the impedance itself stays continuous. Momentum conservation and reciprocity are related ideas in other contexts, but they’re not the primary principle describing energy flow at a media boundary.

When a wave reaches the boundary between two media, energy must be conserved. The energy carried by the incoming wave encounters the interface and is split into two parts: some reflects back into the first medium, and some transmits into the second. In an ideal, lossless boundary, the total energy flux leaving the boundary equals the energy flux arriving—the sum of the reflected and transmitted energy equals the incident energy. This is why conservation of energy is the principle at play for boundary behavior: it governs how energy is partitioned between reflection and transmission.

Impedance continuity isn’t the fundamental rule here; impedance can differ on the two sides, and the mismatch determines how much energy is reflected versus transmitted. The boundary conditions that actually govern the fields require continuity of the tangential electric and magnetic fields, which leads to those reflection and transmission effects, but without claiming the impedance itself stays continuous. Momentum conservation and reciprocity are related ideas in other contexts, but they’re not the primary principle describing energy flow at a media boundary.