What is the difference between emissivity and reflectivity?

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SUMMARY

The discussion clarifies the distinction between emissivity and reflectivity, particularly in the context of materials like painted metal roofs. Emissivity refers to a material's ability to emit thermal radiation, while reflectivity pertains to its capacity to reflect incident light. The conversation highlights that metals, which are good reflectors, often have low emissivity, leading to higher temperatures when exposed to sunlight. This relationship is crucial for understanding thermal management in building materials.

PREREQUISITES
  • Understanding of basic thermodynamics concepts
  • Familiarity with the principles of light and photon interaction
  • Knowledge of material properties, specifically emissivity and reflectivity
  • Basic understanding of thermal radiation and heat transfer
NEXT STEPS
  • Research the emissivity values of various building materials
  • Explore the concept of thermal radiation and its applications in architecture
  • Learn about the impact of surface coatings on reflectivity and emissivity
  • Investigate the role of emissivity in energy-efficient building design
USEFUL FOR

This discussion is beneficial for architects, materials scientists, and engineers focused on energy efficiency and thermal management in building design.

tom421421
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Hi first time posting so apologies if this is the wrong section to post this in!

I'm having a bit of trouble defining the difference between reflection and emission on the atomic level. As far as I can see both just essentially involve excitation of an electron by a photon, followed by de-excitation and re-emission of a photon at a different wavelength. The macroscopic situation I'm trying to apply it to is a painted metal roof in the sunlight. The roof is obviously going to get hot if it has poor emissivity and will be cooler if it has better reflectivity. If the two were the same then a material that was a good emitter would also be a good reflector. This is not the case with a metal, for example, as it is good reflector but gets very hot in the sunlight and must therefore be a poor emitter.

Anyway, I've obviously made a mistake in one of these assumptions so if anyone can let me know where it would be much appreciated!

Cheers!
 
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I'd like to bump this post because I have been struggling with the same concept. If reflection and emission could be considered the same phenomenon, then it would follow that both metal roofs are reflecting the same amount of energy, but the cooler roof is reflecting ( or emitting ) more of that energy in the visible light spectrum.

I am very unknowledgeable, all feedback appreciated
 

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