Lately we’ve been spending a lot of time learning about the planets in our solar system, and thinking about the universe at large. New star systems are being discovered all the time, and it seems like every few months, we’re hearing about another new Earth-like planet! We have some ideas as to what the atmospheres may be like on these planets, and we know a lot about what makes up the planets and moons in our own solar system. The big question we had was, if we haven’t been to these planets and other terrestrial bodies, how on Earth do we know what they’re made of?
I didn’t know the answer to this question, but I did know someone who could help us out! After discussing the topic with a group of physicists in Reddit’s online discussion forum, I decided to put some contacts to good use. A good friend of mine is a physics professor at a local university here in Phoenix. After an exchange of emails, she graciously called me and stayed up until 1:00 in the morning, talking about the spectrum of light, exchanges of energy with electrons, the release of photons (light!), and how we use it to identify the composition of planets and stars!
(Note: The project and tutorial are at the bottom of this post. Follow the explanation of light, physics, and spectra, and make your way to the CD Spectrometer tutorial below!)
So, before we get to the answer to our question, we need to understand how light works. We’ll start with the structure of an atom. Atoms consist of protons, neutrons, and electrons. The protons and neutrons are clustered together within the center of the atom, otherwise known as the nucleus. The electrons are bouncing around everywhere within the electron cloud.
Electrons have different levels in which they can go. These are called shells, and you can think of them as the floor levels between flights of stairs. As with climbing stairs, it takes a lot of energy to get to the next level. Usually, an atom’s shells are filled with a set amount of electrons (the first shell holds 2, the second holds 8, and so on), and the next level can’t be filled until the previous level is filled completely.
However, sometimes electrons can jump levels! They get a ton of energy, usually by heat, electricity, or nuclear fusion (as in the case of stars!), which makes the electrons get excited. Buzzing with excitement from all of that extra energy, they get pushed up to the next shell!
Shortly after they bounce levels, the electrons come crashing back down. After all, they only jumped up because of that extra push of energy, not because the levels below them were completely filled. So, what happens to the energy that pushed them up there in the first place? Well, when the electron falls back down to its previous level, that energy gets blasted off as a photon… it becomes light!
So, you can think of it all like kids jumping in a stairwell. The kids are the electrons and they’re bouncing around like crazy because that’s what electrons (and kids!) do. Some of them get way too excited and trampoline jump all the way to the next level! Then when they come crashing down again, light explodes out of their feet!
Okay, so now we have a basic understanding of how light works. But how does this help us know what elements exist on other planets? The answer has to do with rainbows! This is the visible spectrum of light that forms when light is broken into its colorful parts by something else. When astronomers are looking at other planets or stars, they use an instrument called a spectrograph to read what colors or wavelengths can be seen from an object.
There are two basic kinds of spectra that they are looking for: absorption and emission spectra. In the case of absorption spectra, gases and other elements will absorb certain wavelengths of light, allowing the rest of the spectrum to pass through. Each element absorbed specific wavelengths of light, leaving behind a visible pattern of light waves. Based on the parts of the spectra that are missing, astronomers are able to identify which elements have absorbed the light!
In the case of emission spectra, astronomers are looking for the pattern that is shown when light is emitted from the excited atoms within an element. As stated above, this causes electrons to jump to higher shells, and release photons when they fall back down! These photon are emitted in very specific light waves, depending on how many levels those electrons jumped during their excitement and subsequent fall. Those light waves form the pattern that astronomers look for to identify the elements that they were emitted from!
Scientists measure the wavelengths of light from emission and absorption spectra to determine what the substance is that light has either passed through, or emitted from. These spectra patterns are very specific to each element, as unique as a fingerprint is to a human being! Incredibly, scientists are able to use these measurements to identify the composition of stars and planets from distant galaxies and star systems!
These instruments of astronomical wonder aren’t simply left for scientists to use. You can make your own spectrometer and explore the spectrum of light in your very own home! The best part is, it doesn’t take a lot to make, and you probably already have the materials lying around the house!
MAKE YOUR OWN CD SPECTROMETER!
Pen or pencil
Black construction paper or cardstock
Garden or kitchen shears
Flashlight, light bulb, and any other light sources around your house!
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Starting from the top of your cereal box, use your ruler to mark a line 3 1/2 inches down the side. Mark the same measurement on the other side of the box.
Use your box cutter to cut this section out of your cereal box. Then use your scissors to cut an inch into the box, cutting away the excess cardboard. This will make a little ledge on your cereal box for the CD to sit on.
On the other side of the box, use your box cutter to cut a small slit across the side, approximately 1 inch down from the top. You’ll want the slit to be fairly thin, only about 1 mm high. Cut the slit so that it spans the width of the side of the box.
Use your duct tape to seal the top of the cereal box, so that light cannot penetrate through the opening.
Put on your safety glasses, it’s time to cut your CD in half! Use the garden shears to cut your CD in half. Use your scissors to trim away any excess pieces.
Trim away any excess edges on your CD, so you have a decent half circle with a smooth edge.
With the flat side of the CD facing away from the cereal box, place the CD on your ledge, glossy side facing up. Try to limit the contact of your fingers on the CD as you don’t want smudges to smear your spectra images.
Shine your flashlight through the slit on the other side of the CD box. While holding your flashlight in place, tilt your CD up and down, until you find the angle in which your spectra will show.
Once you have your spectra showing on your CD, duct tape your CD in place!
Once you have your CD securely in place, use your duct tape to create blinders on the edges. This will allow less light to enter your spectrometer from the sides, and provide a sort of shadow box, with which you can view your spectra.
Once you’ve got your CD securely in place, you’re done! You can decorate your spectrometer if you’d like, and make sure you use your duct tape to secure any openings in the box that may cast additional light on your CD. You want your CD to catch only the light coming from the slit in the back of the box, so that it can diffract light and separate it into its wavelengths.
Now that you’re finished putting your spectrometer together, it’s time to have some fun! Go around your house and find as many sources of light as you can! You can use incandescent light bulbs, CFLs, the light from your computer and television screens, the sunlight, a flashlight or nightlight, use anything you can find! When you look at light from different sources, you can really start to get a good idea of the different spectra that can be emitted from each light source!
Most of the spectra we were able to see were absorption spectra. It was really neat to see the rainbow of colors that resulted from elements absorbing wavelengths of light and bouncing the rest around for us to see! Just the four samples above are very different from each other. There is a lot of energy that can be found in the photons, and each source absorbed wavelengths very differently!
Once we were finished examining every light source we could find in our home, it was time to break into some serious spectroscopy. We had a small sampling of gasses in our home, but we wanted to see more elements! For this, we took a trip to our Arizona Science Center. There, they had gas discharge tubes filled with noble gasses! They also had plenty of diffraction lenses, in the form of fun prism glasses! These allowed us to see the spectrum of the elements as they were charged.
Using the Science Center’s equipment, we were able to see spectra of Oxygen, Hydrogen, Helium, Mercury, Xenon, Neon, and more! The best part was, we were surrounded by physics and astronomy enthusiasts, so we were able to talk about spectra and planets for the whole afternoon!
Building our homemade spectrometer brought the world of astronomy and spectroscopy right to our fingertips. What was previously an abstract concept of scientists discovering planets and stars with different gasses and elements, suddenly became a very real activity that we could participate in ourselves! Not only were we delving into some serious science, but we were broadening our understanding of the universe at large.
We were also able to look at our world with a different perspective, and we often found ourselves surrounded by rainbows!
If you would like to learn more about the study of spectroscopy and how it is used in astronomy, here are some helpful links: