The Yellow sodium "Proof"
Have you seen those streetlights that put out yellow light?
 |
Here's a neat demonstration but you need to do it with an adult. Take about a teaspoon of table salt (sodium chloride, NaCl) and put it in a pile in the middle of a heat-proof saucer. Pour about a half of a teaspoon of rubbing alcohol over the salt. Close the alcohol container and move it away. Now CAREFULLY have the ADULT touch a lit match to the salt pile. Turn down the room lights and enjoy the yellow glow! The yellow is from the sodium ions in the salt. See if you can "split" the light through a prism or look at the flame's reflection using a CD. You should see only 2 particularly brilliant yellow lines! |
 | |
You have done a wonderful demonstration in chemistry and optics and explained some history. In the past, tax collectors did this same test on home-made whiskey in local taverns. By law, whiskey had to be at least 50% alcohol and would be taxed accordingly. Whiskey with less than 50% alcohol will not burn when poured on a salt pile. So if the whiskey on the salt did burn it was "100% proved" legal; 50% alcohol. Even today, every bottle of drinking alcohol, (gin, whiskey, vodka, etc.) is labeled in "proof" which is twice the percentage of alcohol content. You "proved" rubbing alcohol is at least 50% alcohol!
OK, so we made very bright yellow flames from burning alcohol on a pile of sodium Chloride (salt). And, if you looked at the light through a prism or reflected in a CD, you saw the light was actually just 2 bright bars of yellow -- no red, green, blue, or anything else.
"Bullets" of light and Electrons
Light (and x-rays, and microwaves, and radiowaves) is made up of little packets, almost like bullets, of energy called photons. Photons are made by electrons and used by one electron to tell another electron about itself. So the yellow light you saw in the sodium (salt/alcohol) flame was from an electron saying,"Hi! I'm an electron in the outermost shell orbiting a sodium atom. A few seconds ago, somebody gave me a whole bunch of energy [that was you, your match, and burning alcohol] and I jumped out to a much bigger orbit! Well, I've cooled down somenow and jumped down to my usual orbit. This yellow photon I'm sending you tells you how far down I jumped."Yup, that's how electrons talk. Now, there was another electron in that sodium atom that started in a slightly closer orbit, jumped up not quite as high, and gave off a slightly lower energy yellow photon when it jumped back down to its usual orbit. These were the 2 yellow lines you saw in the prism or CD.
Every electron is exactly like every other electron. The only thing that determines what color light an electron gives off is how far "down" they jump (Quantum Leap!).
Electrons can't just jump any distance they want. The atom nucleus they orbit has rules about just where it's electrons can orbit. So, a mercury atom usually has 80 electrons in orbit. Each electron is in an orbit unique to mercury. When you see a green photon of exactly the right shade of green (5460 Angstroms) you can say, "Hey! That photon came from an electron around a mercury atom!"
Fingerprints for Gas?
Every kind of atom, from hydrogen (the lightest) to uranium (the heaviest found naturally on Earth), has an absolutely unique photon "signature". If you heat them up, you get photons from their electrons. If you pass these photons through a prism (or a "diffraction grating" like your CD) you get a bunch of bright colored lines. Every kind of atom has a unique set of colored lines. So, if you heat up a lump of anything and look at the "spectrum" of colored lines given off, you can tell exactly what the lump was made of! Nifty! This way of detected what something is made of by looking at it's spectrum of colors is called "spectroscopy".
If you take the light from an incandescent lamp (not a fluorescent bulb!) like the kind Thomas Edison invented and pass that light through a prism (or diffraction grating), you will get your familiar continuous "rainbow" from red, orange, yellow, green, blue, through violet. There will be no bright lines.
Here's the magic. If you take some cold sodium gas (tricky to do but chemists can do this) and put those sodium atoms in a clear glass jar, then put the jar between the incandescent lamp and the prism, what will the spectrum look like? You will see that same old "rainbow" of colors from red to violet except there will be two black lines in the yellow part of the rainbow and the 2 black lines are in exactly the same place as the 2 bright yellow lines when we burned our salt and alcohol!!!!!
Cold atoms absorb the exact same colors of light that hot atoms emit!
Absorption Lines
So far we've learned how every element has a unique set of bright, colored spectral lines that are given off by heating up that element. Once this element cools down it absorbs exactly the same colored spectral lines.These lines are called Fraunhofer lines after Joseph Fraunhofer who, in 1817, was the second scientist to observe them. If there was justice in the world, they would be called Wollaton lines after the scientist who discovered them in 1802.
The spectrum ("rainbow") of the Sun has a whole bunch of dark absorption lines from (relatively) "cool" gas in the Sun's outer atmosphere, the thin gas between the Sun and the Earth, and the gas in our atmosphere.
Since we know what is in our atmosphere, when we look at the dark spectral lines in the spectra from distant stars, we can tell what kind of elements are in that star. We can tell if the star is "old" or "young" or made from the gas from a previously exploded star (just like you!).
But how can we use these Fraunhofer absorption lines to figure out how far away a distant galaxy is from us? And how do we know about the "Big Bang", and how we can tell if another star has planets orbiting it?
So far we have burned alcohol on salt to get yellow flames. We learned that every element will turn to a gas if heated hot enough and that glowing gas gives off very specific colors that show up as bright "bars" if you pass the light through a prism. We also learn that if we pass white light though "cold" gas and then pass the light through a prism, you will see BLACK bars in exactly the same place in the "rainbow" where the BRIGHT bars were when the gas was hot. These are called Fraunhofer lines. Boy, we know a lot!
Ok, how does that help us tell how big, how old, or how dense the Universe is?