Reflection vs absorption and emission

  • #1

Main Question or Discussion Point

Greetings,

Is there a difference between reflection of radiation and emission after absorption? What causes this difference?

Another related question: According to wikipedia: "Earth's surface and the clouds absorb visible and invisible radiation from the sun and re-emit much of the energy as infrared back to atmosphere" (source: https://en.wikipedia.org/wiki/Infrared#Earth_as_an_infrared_emitter)

Does the earth actually emit some of the absorbed visible sunlight as infrared, or is it only infrared part of solar radiation that it simply emits back as infrared? What causes any object to emit radiation at a different frequency than the frequency it absorbed?

Thank you for reading!
 

Answers and Replies

  • #2
DrClaude
Mentor
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Is there a difference between reflection of radiation and emission after absorption?
Yes.

What causes this difference?
I don't know how to explain it simply. Looking at https://www.physicsforums.com/insights/do-photons-move-slower-in-a-solid-medium/ might give you some clues; the interaction of light with solids is somewhat complicated.

Does the earth actually emit some of the absorbed visible sunlight as infrared, or is it only infrared part of solar radiation that it simply emits back as infrared? What causes any object to emit radiation at a different frequency than the frequency it absorbed?
The Earth emits its own radiation, as any warm body will (look up blackbody radiation). Light will be absorb by a given degree of freedom (electronic, vibrational, etc.), then the energy will be redistributed to other modes by internal processes (without emission of radiation), eventually new radiation will be emitted.
 
  • #3
sophiecentaur
Science Advisor
Gold Member
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Is there a difference between reflection of radiation and emission after absorption?
Yes, a great difference. Reflection is a coherent process and specular reflection at a perfectly flat surface will give you an Image. If every photon is absorbed and re emitted by an individual molecule, there cannot be a coherent image because the phases of all the wavelets are all randomised due to the way QM works.
In real life there will be a mixture of the two effects and some photons may interact with individual molecules causing frequency selective loss - which will make an object look coloured, as well as giving good reflections off a surface (i.e. you can see and identify the surface and objects reflected in it).
 
  • #4
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In general the radiation which is emitted by objects is called temperature radiation and has a spectrum of frequencies which vary in a continuous manner. The dominat wavelength shifts to lower side as the temperature rises. The temperature of earth is such that the dominant wavelength is in Infrared region. Please read about balck body radiations or temperature radiation. these are not due to atomic transitions of electrons, which give discrete spectrum certain frequencies are emitted and the frequencies which lie in between are not emitted.
More after you ask some more related questions after reading about it.
 
  • #5
Thank you, people. Your posts have been very helpful! Appreciate it a lot!
 

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