If a surface has reflectivity 0.2 and transmittance 0.0, what is its emissivity?

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Multiple Choice

If a surface has reflectivity 0.2 and transmittance 0.0, what is its emissivity?

Explanation:
The key idea is that a surface balances the incident infrared energy among reflection, transmission, and absorption, and emissivity equals absorptivity for a surface in thermal equilibrium. Since transmittance is zero, the surface is opaque, so all non-reflected energy is absorbed. The absorptivity is 1 minus reflectivity and transmittance: 1 - 0.2 - 0.0 = 0.8. By Kirchhoff’s law, emissivity equals absorptivity, so the emissivity is 0.8.

The key idea is that a surface balances the incident infrared energy among reflection, transmission, and absorption, and emissivity equals absorptivity for a surface in thermal equilibrium. Since transmittance is zero, the surface is opaque, so all non-reflected energy is absorbed. The absorptivity is 1 minus reflectivity and transmittance: 1 - 0.2 - 0.0 = 0.8. By Kirchhoff’s law, emissivity equals absorptivity, so the emissivity is 0.8.

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