# Why are absorption spectra continuous?

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• Cardinalmont
In summary, absorption spectra are mostly continuous, but in the case of gases, they can also be discrete. The energy of each photon determines its wavelength and thus its color. An absorption spectrum should only consist of the specific wavelengths corresponding to the possible discrete energy levels of an electron. Solids and liquids often have absorption bands due to the proximity of other atoms influencing energy levels. The continuous spectrum is the spectrum of the source, while the absorption lines are "holes" in the source spectrum. They are only visible where the original spectral level is high enough.

#### Cardinalmont

Gold Member
It doesn't make sense to me that absorption spectra are (mostly) continuous.
Here are my beliefs. Please tell me which piece/pieces is a/are misconception(s).

1) When light is absorbed, the energy is used to excite an electron to some discrete energy level.
2) To get to this discrete energy level a discrete amount amount of energy must be absorbed
3) Only certain quantities of energy can be absorbed. Slightly too much or slightly too little energy would hypothetically excite an electron to a non-integer state, which is impossible.
4) The energy of each photon determines it's wavelength and thus its color.

Therefore,
5) An absorption spectrum should only consist of the specific wavelengths which correspond to the possible discrete energy levels that an electron can jump up when absorbing the energy

ex.)
n=1 → n=2 Corresponds to ONE specific wavelength
n=3 → n=7 Corresponds to ONE OTHER specific wavelength

I understand that this sort of reasoning is true in regards to emission spectra. I just don't understand why it's different. Thank you for your help.

That depends on the material.
Gases usually have a discrete absorption spectrum.
Solids and liquids often have absorption bands because they have so many states so close together that they form bands. The proximity of other atoms influences the energy levels.

Twigg, Cardinalmont and (deleted member)
mfb said:
Gases usually have a discrete absorption spectrum.

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• Absorption+Spectrum+of+Hydrogen+Gas.jpg
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Cardinalmont said:
Clearly a discrete absorption spectrum. The continuous spectrum is the spectrum being passed through the gas. The absorption is the missing wavelengths.

Cardinalmont
Cardinalmont said:
Yep. Five discrete absorption lines here.

edit: @Orodruin beat me posting.

Ahaha I can't believe I've been misunderstanding this for so many years! Thank you all for pointing this out. I find solace in knowing that my understanding of the physics was correct, just not my understanding of the image.

sophiecentaur and (deleted member)
Cardinalmont said:
Ahaha I can't believe I've been misunderstanding this for so many years! Thank you all for pointing this out. I find solace in knowing that my understanding of the physics was correct, just not my understanding of the image.
I believe this is one of those aha-moments
It is obvious once you know how to think about it but if you start with the wrong interpretation it can be difficult to drop it unless it is pointed out.

dlgoff and Praveen Mani
Out past 700nm is lower than all five absorption lines. Does that mean it absorbs even more out past 700 nm?

Savid said:
Out past 700nm is lower than all five absorption lines. Does that mean it absorbs even more out past 700 nm?
I can't understand what you are implying here. The continuous spectrum is the spectrum of the source. The absorption lines are 'holes' in the source spectrum. They are only visible in the image where the original spectral level is high enough. There is much less energy at either ends of the black body radiation curve and the image just doesn't show it. With the right equipment, you could identify those lines too. IR absorption line spectra are 'a thing'.

Savid said:
Out past 700nm is lower than all five absorption lines. Does that mean it absorbs even more out past 700 nm?
No. It means that the source spectrum has a low intensity there. An absorption line is a discrete frequency where the medium the light is passing through absorbs the light.