Topic 3.11: Spectroscopy and the Electromagnetic Spectrum

Spectroscopy: four lasers make a star

This is the top of Cerro Paranal, a mountain in Chile’s Atacama Desert, one of the darkest and driest places on Earth, where astronomers built four of the biggest telescopes in the world. But the air above the mountain never stops churning, and it smears every star into a shimmering blob, which is the same thing that makes stars twinkle. So they do something that sounds completely made up. They make their own stars.

Those four beams shoot about ninety kilometers straight up, to a thin layer of sodium atoms that meteors leave behind when they burn up in the sky. The sodium takes in the laser light and gives it right back out, so it glows, and to the telescope it looks like four new stars just appeared. The telescope watches those stars and bends a mirror up to a thousand times a second to undo the blur, and the blobs snap into sharp, clean points.

Now imagine they swapped that yellow laser for a green one. Green light carries more energy than yellow, packet for packet, so you’d expect it to work even better. So, what do you think? Switch on the green laser. What would it do to the star?

The green beam goes straight up through the sodium without making a star at all. Crank it up to full power, and there’s still nothing. A red laser would be just as useless.

Only this exact shade of yellow, five hundred eighty-nine nanometers, brings the stars back. So that’s the mystery. You’ll work out the exact color it takes to make one of these stars. And you’ll see why a brighter laser of the wrong color never could.

And along the way you’ll find out what each kind of light, from microwaves to ultraviolet, does to a molecule, and then turn the whole idea around and use light to measure how much blue dye is in a sports drink without tasting a single drop. That yellow beam crossed ninety kilometers of air, and almost none of it got soaked up until it reached the sodium, so different stuff catches different light, and that’s where we’ll start.

The electromagnetic spectrum

Light travels as a wave, and the distance from one crest to the next is its wavelength. Line up all the light there is, from the longest waves to the shortest, and you get the electromagnetic spectrum.

Microwaves are millimeters to centimeters long, infrared is down around millionths of a meter, and everything you’ve ever seen fits in this one thin row, from red at seven hundred nanometers to violet at four hundred, with the laser’s yellow right here. Light also comes in tiny packets of energy called photons, and the shorter the wave, the more energy each packet carries.

When a molecule takes in a photon, it gains exactly that photon’s energy, and it only has a few places to put it. It can spin faster, its bonds can stretch and bend harder, or one of its electrons can move up to a higher energy level. Chemists call those rotational, vibrational and electronic transitions, and this is a classic AP setup, where they tell you which light a molecule absorbed and ask you which of those three things happened.

What each kind of light does to a molecule

One reason astronomers picked this mountain is that it’s one of the driest places on Earth. And the ALMA radio dishes in the same desert sit even higher, at five thousand meters, to get above as much water vapor as they can. To see why, let’s shrink down into the air, blown up about a hundred million times.

It’s mostly nitrogen and oxygen, and the laser’s yellow photons slip right past them. Here’s a carbon monoxide molecule, with a ring on its oxygen so we can follow it, and right now it isn’t spinning at all.

Send it a microwave photon with a wavelength of two point six millimeters, and it swallows the photon and starts to spin, while the bond stays exactly the same length. That’s a change in its rotational energy level.

Nitrogen lets the same microwave fly straight through, because not every molecule can absorb every photon. Carbon dioxide takes infrared instead. At fifteen micrometers it bends harder, with the oxygens swinging one way and the carbon the other, and at four point two six micrometers one bond stretches while the other squeezes. Both of those are vibrations.

Now you’re in charge of the light. Pick the light that makes carbon dioxide bend, and send it in. Water does both, spinning with microwaves and bending with infrared, so water vapor steals some of the infrared and microwave light coming from space, and that’s why these telescopes live in the desert.

Now let’s ride the beam up ninety kilometers to the sodium. When a yellow photon arrives, it’s the electron cloud that changes, because one of the atom’s electrons just moved up to a higher energy level. So microwaves make molecules spin, infrared makes their bonds vibrate, and visible and ultraviolet light move electrons up to higher levels.

Read a wavelength the way the AP exam asks

It all comes down to the size of the steps. Spinning steps are tiny, vibrating steps are bigger, and electronic steps are enormous, with sodium’s more than four thousand times bigger than carbon monoxide’s first spin step.

A molecule only absorbs a photon that matches one of its steps, so microwaves are associated with rotational levels, infrared with vibrational levels, and visible and ultraviolet light with electronic levels. Notice the words associated with, because a molecule doesn’t have to absorb, the way nitrogen didn’t, but when it does, this is the change it goes through. A gas molecule absorbs a microwave photon. What change is most directly associated with that?

Here’s a real one. In 2022 the Webb telescope watched starlight pass through the air of a giant planet seven hundred light-years away, and the light came out missing a little extra at four point three micrometers, the first clear sign of carbon dioxide on a planet around another star. A micrometer is a millionth of a meter, so that’s four point three times ten to the minus six meters, which is infrared, so the carbon dioxide in that planet’s air moved up to higher vibrational levels, the same stretch we just watched.

Saying infrared is heat, so the gas heated up, doesn’t get the credit, because a good answer names the light and the kind of energy level. Let’s try one together, with ALMA, the radio telescope that picks up carbon monoxide in space giving off light at two point six oh millimeters. That’s two point six oh times ten to the minus third meters, a microwave, so the molecule is dropping to a slower spin.

Now I want you to try one on your own. The blue dye in a lot of sports drinks absorbs light at six hundred thirty nanometers, so find its region, and decide what that light does to the dye.