What is the difference between a body that simply reflects the light that falls upon it and a body that absorbs and emits it (like a black body in thermal equilibrium)?

How can one experimentally distinguish these two scenarios by simple measurements? (I think that reflected and transmitted light do not change the temparature of the body where as emitted and absorbed light do. Is this correct and is it possible to experimentally tell the difference between emission and reflection using this information?)

And what are the mechanisms behind reflection and emission? ( I think emission is related to the electron transissions. But how does a photon decides whether to be reflected or transmitted if it is not absorbed ? )


2 Answers 2


When a photon interacts with an atom, three things can happen:

  1. elastic scattering, the photon keeps its energy, and changes angle

  2. inelastic scattering, the photon gives part of its energy to the atom, and changes angle

  3. absorption, the photon gives all of its energy to the atom, and the absorbing electron moves to a higher energy level as per QM

Now, you are asking what the difference is between absorption and reflection, that is elastic scattering:

  1. absorption will change the phase of the photon, the photon will seize to exist and all its energy will be given to the atom, and the atom will emit a new photon or more, that might have different energy level, and phase.

  2. reflection, that is elastic scattering, will not change the energy of the photon, and will give you a real mirror image. Real mirror images not only keep the individual photons' energy, but also keep the photon's phase relative to each other, so the relative angle of the photons will not change. That is how a mirror image is kept.

  3. absorption and re-emission will give you a changed angle, just like reflection, but the change in angle will be different for each photon. The problem with absorption and re-emission is, that the relaxation of the electron might happen in multiple steps, that is called cascades, and so the original photon's energy might be different then the emitted photon's energy, since there will be two or more photons emitted.

Now you asking how you can experimentally distinguish, that is done by checking the original photons' and the reflected/re-emitted light's energy and phase and whether the light coming back is diffuse or specular:

  1. for reflection, the reflected light is always specular, meaning a mirror image, that means that the photons' relative angle is kept the same.

  2. for absorption and re-emission, the re-emission is diffuse, that means that the relative angle of the photons' is random.

You are asking how the atom will decide whether it will reflect or absorb. That is the question, how will the atom decide whether to elastically scatter the photon or to absorb it:

  1. if the photon's energy matches the difference between energy levels of the atom, then the atom will with high probability absorb the photon.

  2. if the photon's energy is lower or higher then the difference between the energy levels of the atom, then the photon might be scattered off the atom with a high probability.

  3. It will be inelastic scattering, that transfers vibrational energies to the molecules if the photon energy is higher that the difference in the energy levels of the atom. It will be an elastic scattering, that is reflection if the photon energy is less than the difference in the energy levels of the atom. Therefore the decision is taken based on the kind of the material and the energy of the photon.


In both cases, light falling on an object is a vibrating electromagnetic field. This makes electrons in the object vibrate and may make them more energetic.

A mirror is a smooth object with free electrons. Light makes them vibrate. Vibrating electrons are accelerated. Accelerated electrons radiate. For a mirror, the radiation is the same as the incoming radiation, but in a different direction. If the surface isn't smooth, it is a lot of little mirrors in many directions.

In some objects light gives the electrons energy, and the electrons pass the energy on to atoms around them. This makes the atoms vibrate more, making them hotter. The energy gets passed around randomly. In some cases, energetic electrons emit radiation. In this case, the emitted radiation is not so directly tied to the incoming radiation. The outgoing radiation depends mostly on the temperature of the object.


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