Question

Imagine you have two different hot, low-density gases, both along your line of sight. For example, one could be a cloud of hydrogen and another a cloud of helium. If you observe the first gas through the second gas, what would you detect with a spectrograph?

a) an absorption spectrum
b) an emission spectrum
c) a continuous spectrum
d) a continuous spectrum with absorption lines
e) a combination of emission and absorption spectra

Answer 1

When observing the first hot, low-density gas through the second, a spectrograph would detect a combination of emission and absorption spectra, revealing both the light emitted by the first gas and absorbed by the second.

Step-by-step explanation:

If you observe the first gas through the second gas using a spectrograph, what you would detect is e) a combination of emission and absorption spectra. In such a scenario, the first cloud of gas, if it is emitting light due to being heated, would produce an emission spectrum. This emission spectrum is characterized by a series of bright lines at discrete wavelengths unique to that element. When this emitted light then passes through the second gas cloud, the atoms in the second cloud can absorb specific wavelengths of light from the emission spectrum of the first gas. This results in an absorption spectrum, consisting of dark lines superimposed onto the emission spectrum.

The final detected spectrum is thus a composite of both emission and absorption spectra. This combined spectrum can be used to identify the presence and composition of the gases in both clouds, based on the patterns of emission and absorption lines.