# Why are absorption spectra continuous?

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.

mfb
Mentor
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.

Praveen Mani and Cardinalmont
Gold Member
Gases usually have a discrete absorption spectrum.

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Orodruin
Staff Emeritus
Homework Helper
Gold Member
Clearly a discrete absorption spectrum. The continuous spectrum is the spectrum being passed through the gas. The absorption is the missing wavelengths.

Cardinalmont
dlgoff
Gold Member
Yep. Five discrete absorption lines here.

edit: @Orodruin beat me posting.

Gold Member
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.

Praveen Mani
Orodruin
Staff Emeritus