Question

Light from a fluorescent lamp is observed through a cloud of cool nitrogen gas. Again, two students are having a discussion about the kind of spectra that they would see.

Student 1: We would see absorption line spectra and the missing lines would correspond to the light from the fluorescent lamp.

Student 2: I disagree. We would see an emission line spectrum corresponding to nitrogen. This would happen because the nitrogen gas would absorb some energy from the fluorescent lamp and would reemit this energy which would result in an emission line spectrum.

**** Which student, if any, do you agree with and why?

Answer 1

In the given scenerio, when the cool nitrogen comes in contact with the photons of visible light, the electrons will absorb the certain wavelength and we would see spectra of wavelengths which are not absorbed. Therefore, we will see absorption line spectra. But let us not arrive at a conclusion yet.

The electrons in the gas clouds that create absorption lines should also eventually fall back down to the ground level, so they should also be emitting photons with exactly the same wavelengths as the absorption lines. They do this, but the reason we still observe absorption lines is because the re-emitted photons can be emitted in any direction, while the absorption only occurs along our line of sight.

Since, in the question it is mentioned that light from a fluorescent lamp is observed through a cloud of cool nitrogen gas, therefore I believe the line of sight is maintained.

Although the student 2 is not completely wrong but according to the reasons given above student 1 is absolutely correct. So, I would agree with Student 1.