No reflection will occur if the two media have _______.

• A. Identical impedances
• B. Slightly different impedances
• C. Very different impedances
• D. Similar densities

Answer: (A)

When a wave encounters the boundary between two media, whether anything is reflected depends on how closely the media’s impedance matches. Impedance in this context is the property that links pressure (or electric field, for light) to the particle motion; for acoustics, it’s Z = ρc, where ρ is density and c is wave speed. If the impedances of the two media are the same, there’s no abrupt change at the boundary, so no portion of the wave reflects back; all of it transmits into the second medium.

If the impedances differ even a little, some of the wave is reflected. With very different impedances, most of the wave is reflected and only part transmits. Simply having similar densities isn’t enough to guarantee no reflection, because impedance depends on both density and how fast waves travel in the medium; two materials can have similar densities but very different wave speeds, leading to a boundary mismatch.

So the condition that yields no reflection is identical impedances.