At the boundary between two media if the reflected and transmitted intensities are added, the result must be the ______.

At the boundary between two media if the reflected and transmitted intensities are added, the result must be the ______.

• A. Incident intensity
• B. Transmitted intensity
• C. Reflected intensity
• D. Total intensity after boundary

Answer: (A)

Energy conservation at an interface is what this question hinges on. When a wave hits the boundary between two media, the energy it carries must go somewhere: part can reflect back into the first medium, and the rest can transmit into the second medium. If the boundary is ideal and there is no absorption or other loss mechanisms, all of the incident energy is accounted for by these two parts. In other words, the incident intensity equals the sum of the reflected intensity and the transmitted intensity. This is why the correct statement is that the reflected plus transmitted intensities add up to the incident intensity. If there were absorption or scattering at the boundary, the sum would be less than the incident intensity, with the difference representing energy lost to the medium.

Energy conservation at an interface is what this question hinges on. When a wave hits the boundary between two media, the energy it carries must go somewhere: part can reflect back into the first medium, and the rest can transmit into the second medium. If the boundary is ideal and there is no absorption or other loss mechanisms, all of the incident energy is accounted for by these two parts. In other words, the incident intensity equals the sum of the reflected intensity and the transmitted intensity.

This is why the correct statement is that the reflected plus transmitted intensities add up to the incident intensity. If there were absorption or scattering at the boundary, the sum would be less than the incident intensity, with the difference representing energy lost to the medium.