An object has a reflectivity of 0.20 and a transmittance of 0.30. Its emissivity is:

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Multiple Choice

An object has a reflectivity of 0.20 and a transmittance of 0.30. Its emissivity is:

Explanation:
In infrared interactions with a surface, the incoming energy is split into reflected, transmitted, and absorbed/emitted portions. The sum of reflectivity, transmittance, and emissivity must equal 1 (energy conservation). Since the surface reflects 0.20 and transmits 0.30, the remaining fraction is 1 − (0.20 + 0.30) = 0.50. That remaining portion is the emissivity (and, by Kirchhoff’s law, the absorptivity as well) under thermal equilibrium. So the emissivity is 0.50.

In infrared interactions with a surface, the incoming energy is split into reflected, transmitted, and absorbed/emitted portions. The sum of reflectivity, transmittance, and emissivity must equal 1 (energy conservation). Since the surface reflects 0.20 and transmits 0.30, the remaining fraction is 1 − (0.20 + 0.30) = 0.50. That remaining portion is the emissivity (and, by Kirchhoff’s law, the absorptivity as well) under thermal equilibrium. So the emissivity is 0.50.

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