Absorption/Emission Spectra Question

[Sherlocked8]

Two experiments provide an emission and absorption spectrum for hydrogen. The emission spectrum is found by analyzing the light from hydrogen gas that is heated to high temperatures, while the absorption spectrum is found by passing incandescent light through hydrogen gas. Which of the best describes the absorption spectrum if it is obtained at room temperature?

a. The absorption spectrum contains all the same lines as the emission spectrum
b. The absorption spectrum is missing some of the lines that the emission spectrum has
c. The absorption spectrum matches the emission spectrum, and even has some extra lines
d. The absorption spectrum contains all the same lines as the emission spectrum, but they are phase-shifted

I answered "a" for this question because I thought absorption and emission spectra are supposed to look identical for the same gas but the correct answer is "b". Can anyone explain why this is?

---

Members don't see this ad.

 

[chemist16]

They are supposed to look "identical" in a way. Think about how emission and absorption spectroscopy works. Heating a gas to high temperatures causes population of excited electronic states. These states can relax back to the ground state and emit a photon at a characteristic wavelength (among other possible routes back to the ground state including internal conversion, etc. - for simplicity, I'll focus on fluorescence here). That's why you get an emission spectrum of lines at specific wavelengths. Absorption is the exact opposite. You pass light through a cool gas and it absorbs characteristic wavelengths of light. In fact, these characteristic absorption wavelengths are the same as the characteristic emission wavelengths because the electronic energy level spacings in a given object are fixed. But, here's the key difference. I just said that the gas absorbs characteristic wavelengths. So instead of getting spectral lines of emission, you get black lines in your spectrum at the characteristic wavelengths for absorption spectroscopy.

One minor point (hopefully it doesn't confuse you further - if you think it will, stop reading here): you might think that when the cool gas absorbs the light, it becomes excited and again fluoresces. This indeed occurs, but it does not emit at the same wavelength it absorbed at - that's why you get the dark spectral lines characteristic of absorption spectra. This is the so-called "Stokes shift."

So, it's "B" because the absorption is actually missing the lines that the emission spectrum has - they're polar opposites.

 

[Sherlocked8]

They are supposed to look "identical" in a way. Think about how emission and absorption spectroscopy works. Heating a gas to high temperatures causes population of excited electronic states. These states can relax back to the ground state and emit a photon at a characteristic wavelength (among other possible routes back to the ground state including internal conversion, etc. - for simplicity, I'll focus on fluorescence here). That's why you get an emission spectrum of lines at specific wavelengths. Absorption is the exact opposite. You pass light through a cool gas and it absorbs characteristic wavelengths of light. In fact, these characteristic absorption wavelengths are the same as the characteristic emission wavelengths because the electronic energy level spacings in a given object are fixed. But, here's the key difference. I just said that the gas absorbs characteristic wavelengths. So instead of getting spectral lines of emission, you get black lines in your spectrum at the characteristic wavelengths for absorption spectroscopy.

One minor point (hopefully it doesn't confuse you further - if you think it will, stop reading here): you might think that when the cool gas absorbs the light, it becomes excited and again fluoresces. This indeed occurs, but it does not emit at the same wavelength it absorbed at - that's why you get the dark spectral lines characteristic of absorption spectra. This is the so-called "Stokes shift."

So, it's "B" because the absorption is actually missing the lines that the emission spectrum has - they're polar opposites.

Then wouldn't the correct answer be that the absorption spectrum is missing "all" of the lines from the emission spectrum, instead of just "some" of them as it states in answer choice B.

 

[chemist16]

That's also what I would say, but that's not an answer option. So the best answer choice appears to be B. The only way around it I can think of is if the Stokes shift just so happens to occur such that the emission does happen at some other wavelength that appears on the heated gas emission spectrum.