“The light we observe, for example the sun’s light, consists of light of various wavelengths. Also, substances have the property of absorbing light of a specific wavelength. So, if you observe the spectra of distant star’s light on earth, you can see which wavelengths are absorbed, so you can see the components of the interstellar gas between the star and the earth.
This time I used a mini light bulb instead of the sun, a chemical liquid instead of the interstellar gas, and a photodiode instead of the earth observer.”