Question Video: Determining Gas Components Using Absorption Spectra Physics

Video Transcript

An astronomer looks at the spectrum of light from a distant star. Between Earth and the star is a large cloud of dust and gas. The star emits continuous-spectrum white light, but some of the light is absorbed by the cloud. The figure shows the spectrum of light that the astronomer observes as well as the absorption spectra of several pure elements. Which of the five elements shown does the interstellar cloud contain?

Whenever we think about spectra, generated by pure elements or molecules, we should keep in mind the emission and absorption spectra of atoms and molecules are unique. This is because the spectra produced by atoms and molecules are the result of energy level transitions of the electrons within those atoms or molecules. Since every kind of atom or molecule has a unique energy level structure, the spectra produced by those atoms or molecules are unique. This also means that by comparing spectra, we can determine what atoms or molecules make up a particular sample.

In particular, when we have a mixture of atoms or molecules, the overall spectrum will be a combination of the individual spectra of the atoms or molecules that make up the mixture. It is important to understand that this is specifically true of a mixture, like when we mix hydrogen and oxygen. But if those hydrogen and oxygen react to make water, the resulting spectrum will change because the electron energy levels of water are different than those of either hydrogen or oxygen.

Anyway, what we now need to do is compare the five spectra we are given to the observed spectrum. Now, these are absorption spectra, which means the features characteristic of each element are the dark lines in each spectrum. If these were emission spectra instead of absorption spectra, the features characteristic of each element would be exactly the same lines in exactly the same places. However, we would see bright emission lines, and the spaces in between would be dark. This is because atoms and molecules emit light at exactly the same wavelengths as they absorb light.

Anyway, what we need to do is compare the dark lines from the five reference spectra to the dark lines in the observed spectrum.

Let’s start with one option we can eliminate almost immediately. Nitrogen has a large cluster of lines in the red portion of the spectrum. This cluster is completely absent from our observed spectrum. Actually, there are a number of features of the nitrogen spectrum that are absent from our observed spectrum, like this other cluster of lines in the red region or this double line in the green region among others. Any one of these is sufficient to tell us that our observed spectrum does not contain nitrogen.

Similarly, carbon has a very wide absorption that is also absent from our observed spectrum. So carbon is not an element in our cloud. And again, any of the lines from carbon that are missing from our observed spectrum are sufficient to tell us that carbon is not present.

We can also eliminate oxygen the same way. Oxygen has several features in the orange and green portions of the spectrum that are simply absent from the observed spectrum. So oxygen is not one of our elements.

For hydrogen though, the situation is different. Every line in the hydrogen spectrum matches up exactly with one of the lines in the observed spectrum. So hydrogen is one of the gases that makes up our cloud.

When we look at helium, we see that each of the lines in the helium spectrum matches up exactly to one of the lines in our observed spectrum as well. Therefore, we know helium is also one of the elements in our cloud.

Finally, we see that with hydrogen and helium we have accounted for all of the lines in our observed spectrum, which means we have accounted for the entire spectrum, and the elements contained in our interstellar cloud are hydrogen and helium.