I am a high school student and I am very confused in absorption and emission spectrum of gases, for e,g take hydrogen at room temperature for simplicity, so that we can talk in terms of Bohr's model of atom

1)We know that when the white light is passes through it, some of the wavelengths will get absorbed by it and electron will be excited to higher energy level, but we also say that when it will return back it will emit he photon of same wavelength, so my question is would these both happens just one after another? i.e will the electron return just after it? but if that's the case it would re-emit is simultaneously so we shouldn't see the dark lines which we usually see?

2}we also know that the any body at any temperature also radiates and if it is at room temperature most of the radiation will be in infrared region, but in this case we are observing that hydrogen will give the photon of lower wavelengths also like when electron would return back from 1st excited state to ground level it will emit radiation of about 122.5nm which lies in visible range, so how is that possible? it is not in accordance with what we see that all bodies at room temperature mostly radiates in infrared region?

please explain it using simple language, which don't involve so many complicated terms because I am at high school level.


1 Answer 1


1} For the absorption spectrum, you have to shine a light through the gas. Light usually comes from one direction. Hydrogen will absorb specific wavelengths from that light and get promoted to an excited state. It will then return to the ground state and emit that light in all directions. So if you look at the light shining through the gas, you see the missing light as dark lines in the spectrum. If you look in other directions you will see the emitted light.

The sky is blue for a somewhat related reason. As light from the sun travels through the atmosphere, some of it is scattered in all directions from small particles. This is Rayleigh scattering. More blue is scattered than other colors. So if you look directly at the sun, it is less blue than it would appear in space. If you look in other directions, you see light that was headed straight away from the sun, but has now been scattered in another direction.

2} Thermal radiation from room temperature hydrogen isn't just caused by promoting electrons to higher energy orbitals. H$_2$ has a molecular bond, which has vibrational and rotational states. See What are the various physical mechanisms for energy transfer to the photon during blackbody emission? for information about other mechanisms.

  • $\begingroup$ Can u please explain , the fraunhofer lines also, i.e we see some dark lines in sun's spectrum, My teacher told me that this is because some wavelengths are absorbed by the chromosphere, but If I go by your answer then if the chromosphere gases have absorbed them they will surely emit them in all directions but ,the radiations from photosphere is already spreading in al directions then why do we see those dark lines if chromosphere is also emitting them in all directions? $\endgroup$ Feb 21, 2021 at 6:42
  • $\begingroup$ ,continiuing, If it is true that they will absorb it and emit the same wavelengths in all directions therby reducing the intensity of them, then suppose we have a green leaf, it is looking green because it is reflecting green and absorbing other wavelengths, but if we go by your answer then it would also emit other colors which it has absorbed, if its true then it should look white? but it didn't look white because I think it is absorbing them but emitting the energy mostly in infrared region isn't it true? then why do gases emit the same wavelength but solid and liquid doesn't? $\endgroup$ Feb 21, 2021 at 6:57

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