Video: Interactions of Electromagnetic Waves and Matter

Video Transcript

In this video, we’re talking about the interactions of electromagnetic waves with matter. These interactions determine most of what we see with our eyes when we look out at the world. And they also help us understand what will happen when we shine light of a particular wavelength onto some certain material. Now, in order to understand these interactions, we’re going to need to know about the two parts involved, electromagnetic waves and matter.

Let’s consider those one at a time, starting out with the waves. Electromagnetic waves, sometimes called EM waves for short, come in a variety of wavelengths. For example, of these three waves, the one on top has the longest wavelength. And then, the wavelength gets shorter as we go down. The wavelength of an EM wave is important because it tells us how much energy that wave possesses. Here’s how these two things are related. The smaller or the larger the wavelength of the wave, the more or the less energy the wave carries. This means that if a wave has a small wavelength, then it carries a lot of energy. But if it has a large or long wavelength, it has less energy.

So, looking again at our three waves, the one on top has the largest wavelength. But that actually means it carries the least energy. The one on bottom with the shortest wavelength has the most. In general, an electromagnetic wave can have any wavelength. There’s really no upper or lower limit. There are waves, for instance, that exist with wavelengths longer than the wavelength of our pink wave. And there are waves that have wavelengths in between the pink and the green, and between the green and the blue, and waves that have wavelengths that are shorter than the blue.

All these different waves, with all these different wavelengths have been organized into what’s often called the electromagnetic spectrum. This spectrum puts waves with a very small wavelength at one end. And then, as wavelength increases, the ones with a large wavelength go at the other end. It’s basically a way of putting electromagnetic waves in an order according to their wavelength. As we draw out our own version of the electromagnetic spectrum, let’s put the short-wavelength waves on the left side and the long-wavelength waves on the right.

And this little sketch here at the top shows us that wavelength increases as we go from left to right. Knowing all this, we can now sketch in the seven distinct regions of the electromagnetic spectrum. Here are those seven sections. First, gamma rays that have the highest energy and the shortest wavelength. Then, up to X-rays with longer wavelength and less energy, then ultraviolet radiation, visible light, the light our eyes can see, infrared radiation, called IR for short, then onto microwaves, and finally radio waves, the waves with the longest wavelength and the least energy.

Now, we’ve seen that the wavelength of this radiation goes up as we travel from left to right. And based on what we saw earlier about the relationship between wavelength and wave energy, we know that if wavelength increases as we go from left to right, that means that energy decreases as we move that same direction. Or another way to say it is that the energy increases as we go from right to left, as we go towards shorter wavelengths. This means, for example, that infrared radiation is more energetic than microwaves, and that visible light is more energetic than infrared, ultraviolet more energetic than visible, and so on.

And it’s at this point that matter, the stuff that these waves interact with, comes into play. We know that, in general, matter is made up of atoms. Atoms, we can recall, are made up of a core called a nucleus, where the nucleus consists of two different types of particles. Neutrons, what we’ve colored here in green, and protons, charged particles that we’ve colored here in blue. And we know that there’s a third type of particle that helps to make up atoms. These are called electrons. They’re negatively charged particles that rotate, or orbit, around the nucleus.

And it turns out that these electrons can be closer to or farther away from the nucleus, depending on how much energy each individual electron has. For an electron with a very low amount of energy, it would orbit the nucleus very close by. But let’s say that one of these three electrons has a higher energy level than the other two. Well, in that case, that higher energy electron would orbit the nucleus a greater distance away. We would say this electron has a higher energy level than the other two.

Now, that doesn’t mean that this arrangement is fixed for all time. It is possible for electrons to move back and forth in-between energy levels. This movement often happens thanks to an interaction with an electromagnetic wave. Let’s say that we have this situation. Our atom is right over here. And an electromagnetic wave comes to interact with it. When this happens, one of three things can take place. The first possibility is that our wave is transmitted through the atom. In other words, it just passes right through. An example of this is visible light passing through window glass. The light passes right through the glass so our eyes can see it. It’s transmitted.

The next possibility is that our wave is reflected. We know there are some materials, like the glass in a mirror, that are specifically designed to do this, to reflect light. But then, there’s also this third possibility that our electromagnetic wave is absorbed by this atom. When we consider the interactions between EM waves and matter, often it’s this option, absorption, that we’re talking about. Here’s how absorption of this electromagnetic energy that’s carried by this wave can work.

We talked earlier about the wavelength of electromagnetic waves and how that helps us determine the energy carried by the wave. That energy amount is very important. The reason is that for an atom to absorb some electromagnetic wave, in other words, for an electron in the atom to absorb a wave, the energy that the wave carries has to be a match for the energy that the electron can accept. In general, an electron can only accept an amount of energy that lets it move up to another energy level. So, if there isn’t a match in those amounts of energy, the energy in the wave and the energy the electron can accept, then the wave would just pass right through. It would be transmitted.

But if there is a match, the electron is able to absorb the energy in the wave, and then it moves up to a higher energy level. Now, this explains how electrons can move from lower to higher energy levels. But what about the electrons that are already at the highest possible energy in this atom? What if they absorb an EM wave? It turns out that this can happen. And when it does, the electron then has so much energy that it effectively escapes from the nucleus. It’s a bit like if a satellite was orbiting the Earth, and it kept speeding up faster and faster and faster. Well, eventually, it would speed up so fast that it would escape Earth’s gravitational pull.

It’s not gravity that keeps electrons in orbit around the nucleus, but the idea is similar. This electron now has so much energy that it escapes from the atom entirely. At this point, let’s recall a certain scientific term. That term is ion. And ion is a charged particle, or it’s a particle that has a net electric charge. As we think about our atom here, originally, this atom was not an ion. That’s because it had the same number of positive charges, charges in the nucleus, as it had negative charges, electrons.

But now, look at what’s happened. We’ve lost an electron, which has a negative charge, which means that the atom left over now does have an imbalance of electrical charge. There are now more positive charges than negative charges. And so, this atom has become an ion. We could say that it has been ionized. For an atom to be ionized, for an electron to be ejected from it, it takes a lot of energy from an incoming electromagnetic wave. In fact, as we look again at our electromagnetic spectrum, only certain categories of waves on this spectrum are capable of ionizing atoms.

That is, only some of these classes of radiation are high-energy enough to strip an electron off of an atom. Now, if we were to draw a line showing the energy required to ionize or remove an electron from an atom, that line falls somewhere within the ultraviolet span of the spectrum. For any waves in the spectrum that have a shorter wavelength, and therefore higher energy than this, those waves are called ionizing radiation. And we can see that they include X-rays and gamma rays and some ultraviolet rays.

What we’re saying, then, is that all gamma rays, all gamma radiation, has enough energy to strip an electron off of an atom. The same is true for X-rays. All X-rays are capable of this as well. And then, when we come to slightly lower energy ultraviolet radiation, certain wavelengths of ultraviolet light can do this and certain don’t have enough energy to. This means that ultraviolet radiation may be ionizing or it may not be. It depends on the specific wavelength. The reason we’re so interested in whether radiation is ionizing or not ionizing is because this phenomenon can have a significant impact on human health.

Think about the cells in our body. Those cells, of course, are made of atoms. Those atoms start off electrically neutral, but if they’re exposed to ionizing radiation, then they become ions. They become charged particles. When this happens for many of the atoms in a cell, there are three different ways the cell can respond. Number one, the cell can die. This ionization damage can be so great that the cell isn’t able to divide anymore. Or number two, the cell is able to repair itself and go on dividing as usual.

And finally, it’s possible for the cell to mutate, to experience some unintended change in its genetic code. This is perhaps the most dangerous of the three possible outcomes because it can mean that the cell begins to replicate, or reproduce, unhealthy cells. And after enough dysfunctional replication, a tumor can develop. So, ionizing radiation is considered dangerous to our cells. And therefore, we avoid it wherever possible. This is one reason, for example, that sunscreen is to be worn whenever we’re out in direct sunlight for a long time. It’s to protect our skin from the ionizing UV-radiation given off by the sun.

Now, of all the waves on our EM spectrum, we can see that gamma rays have the highest energy, that is the shortest wavelength. And this means that of all the possible kinds of waves that could run into an electron that’s already at the outer energy level, gamma rays are the waves most likely to cause that electron to leave the atom entirely. That’s because they deliver the most energy of any wave. From the perspective of creating ions then, we could say that gamma rays interact with matter more strongly than any other kind of wave.

And when we say that, we mean that these rays have the most energy to deliver to an electron. Like we saw earlier though, ionizing an atom by stripping an electron away is not the only or even the dominant way that electromagnetic waves interact with matter. It’s more common for electromagnetic waves to cause electrons in the atoms to move up to higher energy levels within the atom. And we saw that there needed to be a match between the energy carried by that particular wave and the energy difference between where the electron was and where it would end up.

This tells us that different types of waves — visible light, infrared radiation, microwaves — will be easier to absorb by some materials and harder to absorb by others. In other words, the type of wave that an object absorbs most easily depends on the object. For example, consider putting a bowl of food in a microwave in order to heat it up. When we press start on the microwave, the microwave radiation reaches the food as well as the bowl. But because the particular energy level of these waves is a good match for heating up water, the food, which has water molecules in it, tends to heat up more readily than the bowl, which doesn’t have much water.

The food is able to absorb the energy carried by these waves better than the bowl can. And that’s because the energy carried by these microwaves is a better match for the possible energy transitions in the food molecules than they are for the possible transitions in the molecules that make up the bowl. All this to say, depending on what type of matter we’re considering, different kinds of waves — whether radio waves, or infrared radiation, or X-rays — will interact more or less strongly with that matter. Now, to check our understanding of what’s happened so far, let’s look at an example exercise.

One or more of the following types of electromagnetic radiation have no ionizing wavelengths. Which of these types of radiation cannot be ionizing? a) Ultraviolet. b) Radio. c) Infrared. d) X-ray.

Okay, the first thing we can look at is what is ionizing radiation? What does it mean for radiation to be ionizing? Let’s say we have an atom. And in the core of our atom, we have just as many protons, which have a positive charge, as we do electrons, which have a negative charge, orbiting that nucleus. So, our atom has just as many positive charges as it has negative charges. It has no net charge. Therefore, it’s not a charged particle, and that’s what an ion is. A particle with a net charge.

In order to ionize an atom to give it a net charge, the most common method is by stripping away one of its higher energy electrons. And this is where these different types of radiation get involved. If an electromagnetic wave with enough energy is absorbed by one of these high energy electrons, then that electron can gain so much energy by the absorption that it’s effectively ejected from this atom. Once the electron leaves an atom, that means the atom left behind now does have an overall, or net, electric charge. In other words, it’s an ion. And that ion was created through this absorption process.

Now, the ability of this incoming wave to cause an electron to be ejected from the atom all depends on how much energy the wave carries. There’s some minimum amount of wave energy required in order to do this. This means that not all radiation is ionizing radiation. Not all waves have enough energy to do this. In this exercise, we wanna pick out which of the types of radiation from our list cannot be ionizing, in other words do not have enough energy to eject an electron from an atom.

To see which radiation types those might be, let’s briefly recall the seven categories of radiation which were summarized in the electromagnetic spectrum. If we organized our spectrum so that higher wavelengths were off to the right and higher energies were off to the left, then our seven categories would be ordered like this. Gamma rays on the far left, then X-rays, then ultraviolet light, visible light, the light our eyes can see, then infrared radiation, microwaves, and then finally radio waves with the longest wavelengths of them all.

We noted earlier that it’s the higher energy waves which are capable of ionizing an atom. And it turns out that the cut off energy, the minimum energy required to do this, occurs in the UV part of the spectrum. In other words, for some ultraviolet radiation and for all X-rays and all gamma rays, that radiation is energetic enough to be ionizing radiation. But then, on the other side of this line, the radiation doesn’t have enough energy to strip an electron off of an atom. It can’t ionize an atom. So, now, let’s look at our four options a, b, c, and d.

Option a, ultraviolet is a bit interesting because we see that this is the region of the spectrum where the energy cut off occurs. But look now at the specific wording of our question. Which of these types of radiation cannot be ionizing? Because there are some wavelengths of ultraviolet light which are ionizing, we can’t choose a as one of our answers.

Look, then at option b, radio waves. We see these waves are all the way to the right of our spectrum. They have the highest wave length and the lowest energy. These waves are far to the right of the ionization cut off. So, these cannot be ionizing. And similarly with infrared radiation, which again is to the right of that energy minimum. Our last option, X-rays, indicates wavelengths of light which are all high energy enough to be ionizing. Option d, then, is not an answer that we’ll choose. So, of these options, it’s only radio waves and infrared radiation which cannot be ionizing.

Let’s summarize now what we’ve learned in this lesson. Starting off, we saw that electromagnetic, or EM, waves carry energy. And the smaller their wavelength is, the more energy these waves possess. Then, we talked about matter. And we saw that it’s made of atomic nuclei as well as electrons with different energy levels that orbit the nuclei. We learned that electromagnetic waves can be absorbed by electrons, which raises the electron’s energy level.

If the wave has enough energy, then an electron can leave an atom entirely, creating a charged particle, an ion. The waves that have enough energy to do this are gamma rays, X-rays, and some wavelengths of ultraviolet waves. These are ionizing radiation. And finally, we saw that the type of radiation an object interacts with most easily depends on the energy levels in that object’s atoms.