Introduction to spectroscopy | Intermolecular forces and properties | AP Chemistry | Khan Academy

In this video, we're going to talk about spectroscopy, which is all about the interactions between light and matter. When we're talking about light, we're not just talking about visible light; we're talking about electromagnetic radiation in general. So, what I'm going to do to give us an intuition here is use the PhET simulator by the University of Colorado. I encourage you to go to this URL and try it out for yourself.

What the simulator does is it allows us to essentially see how different wavelengths of electromagnetic radiation can interact with matter—in this case, various molecules. Just to get our bearings, we can click on this light spectrum diagram, and we can see that on this diagram, what people would normally consider radio waves are some of the lowest frequencies and longest wavelengths of light. Then, when you get to higher frequencies, you get to microwaves, and the higher the frequency, that’s also the higher the energy per photon.

Then you get higher frequencies than that and higher energy; that's infrared. Higher frequency and energy than that correspond to visible light—that’s what our eyes can sense. Then you get even higher frequency and more energy; you reach ultraviolet, then X-ray, and then gamma rays. This isn't a linear scale; you can see that this is a logarithmic scale here—this is in powers of 10. We see some pretty dramatic increases in frequency and energy as we go from the left to the right.

But in this video, we're going to focus in particular on microwave, infrared, visible, and ultraviolet wavelengths of electromagnetic light or electromagnetic waves, and think about how they interact with molecules. If we start with microwave radiation, here we have a water molecule—I've picked that right over there. I can get my simulation going; you can see what it's doing. When it gets absorbed, it causes a rotational transition in the water molecule. It makes the water molecule rotate in a different way than it was before.

Then the water molecule can also emit the radiation and rotate differently. You can see it doesn't always do that; there's a little bit of probability involved. But this is actually the basis of how microwaves work. Your microwave oven causes the water molecules to get agitated in a rotational way, which increases the heat in that system.

Now, we could also look at infrared light, which is, once again, important to remember as it gets us into higher frequencies, and see what that does to molecules. Based on this simulation, it looks like the infrared light, when it gets absorbed, causes this water molecule to start to vibrate. Microwave radiation caused it to rotate or to have a change in state of its rotation, while infrared makes it vibrate. We can see that with other molecules as well. Let's say, let's try carbon monoxide. Once again, it's not rotating it; it's causing it to vibrate.

Now, what about visible light? Visible light will have different interactions with different types of molecules, but let's try it out with nitrogen dioxide. There are certain situations where nitrogen dioxide will absorb—that’s when you saw it glowing. What you see when it's glowing is that it's putting electrons into a higher energy state or into a higher orbital. When it stops glowing, it means that those electrons are going back to a lower energy state, and they are re-emitting radiation.

You can see that just now; it's re-emitting visible light in this case in a different direction. When it did that, the electron that was excited went to a lower energy state. Now, what about ultraviolet light, which has even higher energy than visible light? What can that do? Here, we can see that it takes, in certain cases, electrons and is able to excite them so much that it can actually break that bond itself.

Let me keep resetting it so you can actually see it can break bonds. Let’s see what it can do to some ozone. Same thing! It excites it so much that it can actually break the bond. It's exciting an electron so much. I can keep resetting it.

So, the big picture here, the big takeaway: you could have microwave radiation, which tends to change the rotational motion of a molecule. We saw that with the water molecules. You have infrared radiation, which is higher energy and higher frequency, which tends to lead to a change in vibrational motion. Then you have visible light, which can excite electrons, taking them to a higher energy state, and then be re-emitted when the electron goes back to its base state.

Finally, you can have ultraviolet light that is so powerful it can excite electrons so that, in some cases, it can even break covalent bonds.