Video: The Electromagnetic Spectrum

In this lesson, we will learn how to analyze the electromagnetic spectrum by identifying and describing types of electromagnetic radiation and their sources.

13:45

Video Transcript

In this video, we’re talking about the electromagnetic spectrum. This spectrum is a start point for learning about all different kinds of radiation, also known as light. As we’ll see, once we understand this spectrum, then we’ll also understand lots of the physical phenomena going on around us.

We can start off on this topic by considering this question. Where does light come from? Well, for starters, we know that the Sun creates light, and so does a light bulb, as well as a television or a computer monitor. Even though there are lots of different sources of light, there’s a basic physical mechanism that ties all these sources together.

If we go down to the level of individual atoms, that mechanism is the acceleration of electric charge, in particular electrons, as they change energy levels within an atom. When such a transition happens, when an electron moves from one energy level to another, often this is accompanied by the emission of a little packet of light called a photon. When a photon is emitted, its properties, such as its wavelength and its energy level, depend on just how it was produced, how big of a transition an electron went through, or, along a similar line, how much of an acceleration the electron experienced.

Now there’s a very specific reason we’re talking about photons and light. If we were to take a closer look at a photon emitted in a process like this, we would see that this packet of light, this photon, is actually a series of oscillating fields, a magnetic field and an electric field. And talking about those fields, sometimes we abbreviate electric with a capital 𝐸 and magnetic with a capital 𝐵. We can see then that light is an electromagnetic entity. It’s something that’s made of an electric field and a magnetic field. This means that the term “electromagnetic radiation” is just a fancy way of saying light, which means if we understand where light comes from, we know where electromagnetic radiation comes from. And it’s that radiation which is described in the electromagnetic spectrum.

Now going back briefly to this emitted photon, it’s fairly common to represent these photons using a squiggly line, like we drew here. The reason for this is that photons, as we mentioned, have a wavelength associated with them. And as it turns out, the electromagnetic spectrum, which is the spectrum, the collection, of all electromagnetic radiation possible, that means that, at one end of the spectrum, over here at this side say, we have light of very short wavelength. And then as we move from left to right across this spectrum, the wavelength of light involved gets longer and longer. And notice here that we’re using the Greek letter 𝜆 to represent wavelength. That’s a common abbreviation.

Now if you’ve seen a picture of the electromagnetic spectrum before, you’ve likely seen it divided up into different regions. There’s one region for visible light, one region for X-rays, one region from microwaves, and so on and so forth. While it is helpful to think of light of electromagnetic radiation according to these regions or sections, it’s important to realize that physically there is no such separation between different regions.

If we go back to our sketch of the spectrum that we saw on the opening screen, we see that there’s no division between different regions of the spectrum. The wavelengths just smoothly and continuously get longer as we move from left to right. So as far as the spectrum goes, we observe in nature that light can have virtually any wavelength. But for our purposes, to help us understand these different wavelengths, we go back in after the fact and divide up the spectrum into different regions.

In general, the spectrum is divided up into one, two, three, four, five, six, seven different regions. And understanding the spectrum involves being able to recall the name for each one. Knowing these seven names is not as hard as we might think. What we do is we start out with a one type of light we’re sure exists. That’s the light our eyes can see, also known as visible light. This region is at the very center of the spectrum. And it includes all the colors of the rainbow: red, green, blue, violet, et cetera.

If we were to look at the visible colors on either end at the extremes of the visible spectrum, on the long-wavelength end of the spectrum, we would see the color red. And on the short-wavelength end, we would see the color violet. Knowing these two colors and recalling which ends of the visible part of the spectrum they apply to is helpful for knowing the names of the regions on either side of the visible part.

To recall the names of these two regions, it’s helpful to know a little bit about a couple of different prefixes. First, consider the prefix “ultra.” It means beyond. So, for example, an ultramarathon is a marathon that’s even longer than a regular marathon. It’s beyond a marathon.

As we consider the region of the spectrum that has shorter wavelengths than visible light, the one just to the left of it in this sketch, this region gets its name from the fact that it is ultra or beyond violet light. And indeed, ultraviolet is the name of this region of the spectrum. And it’s sometimes abbreviated capital U capital V. These are the types of rays created by the Sun, which are higher energy than visible light. And indeed, that comes from the fact that they have a shorter wavelength. We can see that they’re farther to the left on our spectrum than visible radiation. So let’s add that to our description of what either end of our electromagnetic spectrum involves.

On the left end, we have light with a relatively short wavelength and therefore a higher energy. While on the right end, we have light with longer wavelength and therefore lower energy. So the higher-energy region of the spectrum that’s adjacent to, right next to, visible radiation is called the beyond violet or ultraviolet region.

But then what about the other side of the visible part of the spectrum? This is where our second prefix, “infra,” which means below, helps us. Since the visible light that’s at the edge of this transition between the two regions is colored red, we can name this whole region below or infrared. And indeed, that’s the name of this region of the spectrum, infrared. And often, to represent this region, we’ll see the abbreviation IR, which stands for infrared radiation.

If we keep going beyond the infrared part of the spectrum to longer wavelengths and even lower-energy waves, what we’ve hit on is the microwave region of the electromagnetic spectrum. A good way to recall this name is to realize it’s the name of a device we often use to heat up our food, our microwave. Interestingly, these waves, by which we heat up virtually any type of food, have wavelengths on the order of 10 to the negative two meters or one one hundredth of a meter long.

Then as we move past the microwave region to the longest-wavelength region of the whole spectrum, we encounter the range of radio waves. These indeed are just the sort of waves that are transmitted by radio towers. Using a radio selector, we tune in to different channels of radio wave radiation. These waves are at least a meter long and can be much longer in wavelength than that.

Now as we said, the far right edge of the spectrum has longer wavelengths and correspondingly lower energies to these waves. That means of course that if we travel in the other direction, we’ll have shorter wavelengths and higher-energy electromagnetic radiation.

If we venture out beyond the ultraviolet range and go to a higher-energy region, then we’ve arrived at what’s known as the X-ray region of the spectrum. One characteristic of high-energy radiation like X-rays is its ability to penetrate through matter. We’ve probably all had an X-ray taken of some part of our body, where these waves are high energy enough to transmit through soft tissue and are only blocked or stopped when they reach something very dense like bone. X-ray wavelengths are very small, on the order of the size of an atom, 10 to the negative 10th meters.

But as we can see, there’s an even higher-energy region of the electromagnetic spectrum. The radiation in this part of the spectrum is known as gamma rays. Gamma rays have very short wavelengths, less than 10 to the negative 15th meters. And their most common source is decaying atomic nuclei. When the nucleus of a decaying atom splits, when it breaks apart, often times gamma rays are emitted.

If we fill in the approximate wavelengths of ultraviolet, visible, and infrared radiation, then what we have is a completed electromagnetic spectrum diagram. We have all seven regions arranged in order, left to right, from shorter wavelength to longer wavelength or, correspondingly, from high energy to low energy. This is the spectrum as we’ll often see it.

But it’s also helpful to add a bit of information about where these different types of radiation come from. Even though radiation in general comes from transitioning accelerating electrons, for each of these regions, we can get more specific about the typical mechanism that generates this radiation. If we start at the left side of our spectrum, with gamma rays, the highest-energy radiation, as we said, these are typically generated through nuclear decay, that is, the radioactive deterioration of atomic nuclei.

A standard way to create X-rays is to rapidly decelerate electrons by speeding them up very fast and then having them slam into a stationary target. This is the general mechanism by which X-ray tubes generate X-rays.

When it comes to ultraviolet and visible radiation, the primary source of this light is the Sun. The Sun also creates a good deal of infrared radiation. But it turns out that infrared radiation, or IR, is low energy enough that any object in our surroundings is a source. That’s because this radiation is due to what’s called the thermal motion of atoms and molecules. In other words, just by virtue of being at, say, room temperature, an object will emit infrared radiation.

And then going beyond infrared radiation to microwave and radio waves, this type of light is created by electric currents, whether alternating or direct. For both kinds of current, the wave generation process relies on changes to the current. With alternating current, that change happens naturally. And for direct current, the change happens by turning on and then off over and over again that same direct current. Effectively then, this makes the DC or direct current behave a lot like AC or alternating current.

Now that this chart is complete, let’s look carefully at the screen and do our best to remember what we see on it. And now let’s get a bit of recall practice through an example.

Which of the following could be a source of infrared radiation? A) Alternating electric currents, B) decaying atomic nuclei, C) direct electric currents, D) thermal motion of atoms and molecules, E) none of the answers is correct.

We see that each one of the options A through D is a candidate for being a source of infrared radiation, a particular type of radiation in the electromagnetic spectrum. As we consider which of these four options could be a source of IR, infrared, radiation, let’s start out up at the top with option A, alternating electric currents.

When alternating electric currents are used to generate electromagnetic radiation, what is typically produced from this source is either microwaves or radio waves. This is because the frequency of oscillation of these currents is low enough that it produces these particular types of radiation. We see that not only option A talks about electric currents, but so does option C, but this time in the form of direct electric currents, that is, currents that always move in the same direction.

Even though direct currents do always move the same way, we can effectively turn them into alternating currents by switching these direct currents on and off over and over again. It’s by this mechanism that radio waves can be created. What we’re seeing is that both of these options, alternating as well as direct electric currents, do act as sources for electromagnetic radiation, but not sources for infrared radiation. Instead, they’re typically used to create microwaves and radio waves. So we’ll cross those off our list of options.

Moving on to option B, decaying atomic nuclei, this is a process where an atomic nucleus splits or breaks apart into smaller pieces — that’s called fission — and in the process releases energy through electromagnetic radiation. But the type of radiation typically emitted through this process is gamma radiation, that is, the emission of gamma rays. So once more, this option is a source for a particular type of electromagnetic radiation, but not the type we’re interested in, infrared radiation. So we cross option B off our list too.

Next, we get to option D, the thermal motion of atoms and molecules. Here’s what this option means. Everyday objects such as chairs, tables, and the like, simply by being at room temperature, about 20 degrees Celsius or about 70 degrees Fahrenheit, has enough thermal energy that the atoms and molecules in these objects are in thermal motion. And thanks to this thermal motion, a particular type of radiation is emitted. And this indeed is infrared or below-red radiation. That is, our eyes aren’t sensitive to this particular wavelength of radiation. But nonetheless, it’s there and it’s created by thermal motion of atoms and molecules. This option can be a source for infrared radiation. And therefore, option E that none of the answers is correct is itself not correct. And so our final answer is that thermal motion of atoms and molecules could be a source for infrared radiation.

Let’s take a moment now to summarize what we’ve learned about the electromagnetic spectrum. In this lesson, we saw that, in general, light, which is another name for electromagnetic radiation, is created through the acceleration of electric charge. And that often this acceleration happens in the context of electron transition within energy levels of an atom. We saw that the electromagnetic spectrum organizes all the light that can be produced by the light’s energy or equivalently by its wavelength. And we also saw that the spectrum is divided into seven distinct regions. If we arranged the spectrum from high-energy radiation, that is, radiation with shorter wavelengths, on one end, and low-energy radiation, radiation with longer wavelengths, on the other. Then going from high energy to low energy, these seven regions are gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and then radio waves.

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