I am a high school student and I am very confused in a topic which is related to sun's spectrum, My teacher told me that the dark lines which we see in sun's spectrum are called fraunhofer lines and it forms because the elements in chromosphere of sun absorbs some of the wavelengths, and he also says that fraunhofer lines can be explained by kirchhoff law, but I don't understand how kirchoff law can explain it?

kirchoff law simply states that at thermal equilibrium emissivity of a body equals to its absorptivity( I am also confused in another thing that if emissivity equals to absorptivity only at thermal equilibrium then why do we only use emissivity while writing the formula for rate of heat loss due to radiation?we should separately use absorptivity for absorption part and emissivity for emission part?), what does it has anything to with fraunhofer lines? please explain it in a simple language so that I can easily understand {remember please I am only at high school level}kirchoff law


1 Answer 1


If a material can absorbs more energy than it emits it will heat up, if it can emit more energy than it can absorb it will cool down.

So, at equilibrium (temperature is not changing with time), absorption and emission rates must be the same. This is Kirchoff’s law.

Fraunhofer lines indicate that the light emitted from the sun is being filtered by various elements in the suns’ photosphere. Because this gas is at a lower temperature than the core it can absorb some of the energy, and it will have a lower emission rate.

There is also an effect of light scattering. The photosphere will absorb certain lines, and will remit, but the re-emission will occur isotopically. Therefore the amount of energy along the original path is reduced.

  • $\begingroup$ but where is the kirchhoff law applied in all of this? $\endgroup$ Mar 8, 2021 at 14:24
  • 1
    $\begingroup$ 3rd paragraph, because net energy can be absorbed it implies there is a temperature gradient, otherwise the plasma would emit exactly as it absorbs. $\endgroup$
    – boyfarrell
    Mar 8, 2021 at 16:12
  • $\begingroup$ The temperature gradient is the key thing $\endgroup$
    – ProfRob
    Oct 22, 2022 at 12:35

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