Why are absorption spectra continuous?

  • #1
Cardinalmont
Gold Member
22
4
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.
 

Answers and Replies

  • #2
35,992
12,862
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.
 
  • Like
Likes Twigg, Cardinalmont and (deleted member)
  • #3
Cardinalmont
Gold Member
22
4
Gases usually have a discrete absorption spectrum.

Absorption+Spectrum+of+Hydrogen+Gas.jpg
 

Attachments

  • Absorption+Spectrum+of+Hydrogen+Gas.jpg
    Absorption+Spectrum+of+Hydrogen+Gas.jpg
    33.1 KB · Views: 11,861
  • #4
Orodruin
Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
2021 Award
18,958
8,939
  • Like
Likes Cardinalmont
  • #6
Cardinalmont
Gold Member
22
4
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.
 
  • Like
Likes sophiecentaur and (deleted member)
  • #7
Orodruin
Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
2021 Award
18,958
8,939
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 :wink:
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.
 
  • Like
Likes dlgoff and Praveen Mani
  • #8
1
0
Out past 700nm is lower than all five absorption lines. Does that mean it absorbs even more out past 700 nm?
 
  • #9
sophiecentaur
Science Advisor
Gold Member
27,272
5,960
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'.
 
  • #10
Orodruin
Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
2021 Award
18,958
8,939
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.
 

Related Threads on Why are absorption spectra continuous?

Replies
2
Views
2K
  • Last Post
Replies
1
Views
2K
Replies
8
Views
2K
Replies
3
Views
8K
  • Last Post
Replies
12
Views
2K
  • Last Post
Replies
4
Views
5K
Replies
2
Views
3K
Replies
4
Views
2K
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
0
Views
9K
Top