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Question Video: Comparing the Lines of Two Emission Spectra Physics • 9th Grade

The figure below shows two spectra A and B. Does the 541 nm emission line in spectrum A appear in spectrum B?

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Video Transcript

The figure below shows two spectra A and B. Does the 541-nanometer emission line in spectrum A appear in spectrum B?

The spectra referenced in this question are these two spectra here. And just from looking at them, we can tell that they are emission spectra because they consist of bright lines on a dark background. Each of these bright lines corresponds to one of the discrete transitions that electrons can make between the energy levels in an atom or a molecule. The particular emission line we’re interested in is the emission line with a wavelength of 541 nanometers. And taking a look at the scale at the top of the diagram, we see that 541 nanometers is right about here. So the emission line we’re interested in is this green line.

Remember, emission lines are the result of the energy associated with electron transitions within an atom or molecule. So, even though we are thinking about these in terms of wavelength, the fact that wavelength and energy are directly proportional for light means that thinking about the energy of a spectral line and the wavelength of a spectral line are equivalent.

Anyway, our task is to determine whether this particular spectral line also appears in spectrum B. Looking at the figure, we see that the 541-nanometer emission line in spectrum A corresponds to a dark spot in spectrum B. That is, there is no 541-nanometer emission line in spectrum B. So the answer is no, the 541-nanometer emission line in spectrum A does not appear in spectrum B.

Doing this sort of comparison between two spectra is important because emission and absorption spectra from atoms and molecules are unique to the particular type of atoms or molecules that make up the sample. So the presence of the 541-nanometer line in spectrum A and its absence in spectrum B tells us that there is at least one atom or molecule that is in the sample that was used to produce spectrum A and is not in the sample that was used to produce spectrum B.

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