Video: Infrared Radiation

In this lesson, we will learn how to describe how the temperature of an object and its surface features affect the object’s emission and absorption of infrared radiation.

15:31

Video Transcript

In this video, we get to talk about infrared radiation. And to get started, consider this split screen view of an electric hair dryer. The hair dryer works by blowing hot air out of the nozzle at this end. But of course, if we look at a hair dryer while it’s in operation in our everyday life, we don’t see this hot air coming out that nozzle. In addition, we are unable to see any parts of the hair dryer getting hotter as it does this. But if, instead of looking at the visible light coming off of this hair dryer, we were to look at the infrared radiation coming from it, then in that case, we could see the hot air being blown out by the hair dryer as well as the parts of the dryer that are heating up. And this leads us to our first important fact that infrared radiation, often abbreviated IR, is associated with heat.

Here’s what we mean by that. Imagine that you’re standing by a campfire, warming yourself. Now, looking at this campfire, we know that it gives off visible light. That’s the light that we can see from the flame. But as we stand fairly close to this campfire, we can also feel the heat that the fire gives off. That heat that we’re feeling is not due to the visible light, the yellow and red colors we’re seeing in the flame of the fire. But rather, it’s due to the infrared radiation given off by the fire. So we can see that visible light, the light that our eyes are sensitive to, and infrared radiation are not the same thing. Nonetheless, they are connected. Both of them are examples of electromagnetic radiation.

If we take a look at the electromagnetic spectrum, sometimes called the EM spectrum for short, we see at the middle of the spectrum as it’s been drawn visible light, that’s the light that our eyes are sensitive to, and then just to the right of that infrared radiation. Continuing on to the right, there are microwaves and then, at even longer wavelengths, radiowaves. And then, if we go back to visible light and move left instead of right, we reach ultraviolet radiation, called UV for short, and X-rays and, finally, gamma rays.

What we’re seeing is that as we move from left to right on this spectrum, the wavelength, symbolized by the Greek letter 𝜆, gets longer. And then, as we move right to left, the wavelength gets shorter. Looking back at the visible portion of the spectrum, this part includes all the colors that our eyes can see, red, green, blue, yellow, and so forth, all of the colors our eyes are sensitive to. If we were to go to the far right side of the visible spectrum, we would be looking at light that’s colored red. And the wavelength of red light is approximately 700 nanometres.

Recall that one nanometre is equal to 10 to the negative ninth metres. In other words, a nanometre is one billionth of a metre. This tells us that red light has a wavelength of 700 billionths of a metre. And then, if we move to the right into infrared radiation territory, the wavelength lengthens. It increases. If we go to the far right edge of this part of the EM spectrum, then the wavelength there is 10 to the sixth or one million nanometres. At this point, we can recall that not only is one nanometre equal to a billionth of a metre, but it’s also equal to one millionth of a millimetre. So therefore, we can write 10 to the sixth or a million nanometres simply as one millimetre. And what we found now is the complete wavelength span of infrared radiation, from 700 nanometres to one millimetre.

Now, we’ve said at the top of the screen that infrared radiation is associated with heat. That’s true. But it’s really the infrared radiation to the right-hand side of this range that’s responsible for the heat that we feel. Sometimes this radiation is called far IR, far infrared radiation, or thermal IR. Now, if we think back to that picture we saw on the opening screen of the hair dryer in operation, we can see just how it is that infrared radiation, specifically thermal infrared radiation, can be very useful to look at.

For example, say that you have a job working as a security guard for a department store. As part of your job, you want to monitor activity that happens in the parking lot out front of the store. To do this, you install a few cameras on the store that are able to record the activity in the parking lot. But there’s just one problem with the system as is. The cameras that have been installed are only sensitive to visible light, the light our eyes can see. So during daylight hours, they’re great at monitoring the parking lot. But when the sun goes down and the whole parking lot goes dark, then the whole image recorded by the cameras goes dark as well. And it’s hard to tell what’s going on out there. Well, this is one place where infrared radiation could come in handy.

If we switch out these cameras so that now they’re sensitive to infrared radiation rather than visible light, then in that case, even at night, even when there’s no visible light on this parking lot, when we look at what we could call the IR view of this scene, then we’re still able to see objects, not based on their visible light signature but based on the heat that they give off. For example, this view would show the heat produced by the engine of the car that’s parked in the parking lot. And it would also show the heat given off by the person walking out of the car. By taking this view of things, the infrared radiation view rather than the visible light view, it’s possible to see what’s going on in a given scenario, based on the heat involved rather than the visible light.

Speaking of this difference, it’s a pretty fun thing to compare how a person looks in a visible light picture, a picture we would normally see, compared to an infrared radiation view of that person. Since the IR view is based on the heat given off, we start to notice that often the tips of our noses are colder than the rest of our face, so they appear darker in an infrared radiation view, that our eyes and mouth are warmer. And in general, we get a view for the temperature gradients across a typical person’s face. Now that we’ve considered how objects appear when viewed from this infrared radiation perspective, let’s look at how infrared radiation interacts with materials.

Say that we have some chunk of material here. And we shine infrared radiation on it. Just like with visible light, there are one of three things that this material can do with the light once it arrives. The light can be reflected from the material. It can be transmitted through. And as a third option, the light can be absorbed by the material. So there’re these three options: absorption, taking the light in, reflection, having the light bounce off the material, or transmission, letting the light pass through.

Now, let’s think for a moment not about infrared radiation but about visible light, the light that our eyes is sensitive to. If we look around at our surroundings, we notice that most of the objects we see are not transparent. We can’t see through them. In other words, for most everyday objects, when it comes to visible light, there’s very little transmission going on. Nearly, all of the light is either absorbed or reflected by a material. Well, it turns out that infrared radiation behaves similarly. Certainly there are exceptions. But most of infrared radiation is either absorbed or reflected by objects. We can say that most incident infrared radiation is either absorbed or reflected by a material. So if we neglect any transmission that goes on, assuming that that transmission is negligibly small, then we can start to make a list of factors that affect whether a material absorbs or reflects infrared radiation that hits it.

So let’s make a list of these factors over on the left-hand side of our screen. And as we go along, we’ll see how changes to these factors impacts what goes on here in our sketch. The first thing we can think about when it comes to whether a given material will absorb or reflect infrared radiation is the color of that material. The color of this material we’ve drawn here is white. White, just like with visible light, is very good at reflecting infrared radiation. This means that most of the IR that reach this material would just bounce off of it. The great majority is reflected. And very little is absorbed. On the other hand, if we were to make our material color black, that would mean the material is very unlikely to reflect radiation and very likely to absorb it.

Now, take a quick look at this title here, factors affecting IR absorption and emission. It turns out that these two terms, absorption and emission, are very closely connected. If a material absorbs a lot of infrared radiation, like our black material is here, then it’s also likely to be a very good emitter of infrared radiation. It gives that radiation off as heat. This is why, for example, if you walk over blacktop on a hot summer day, you’ll feel much warmer than if you’re walking on a light-colored pavement. Even though the pavement in the blacktop may be the same exact temperature, the blacktop is much more effective at absorbing infrared radiation and therefore emitting it. And it’s that emission that we sense as heat. So material color plays an important role in how a material absorbs and then emits infrared radiation.

There’s another factor that affects these quantities, called reflectivity. We could think of reflectivity like this. Say that this material is a smooth, highly polished material, such as a mirror. In that case, it has a very high degree of reflectivity. Most of the infrared radiation that reaches it will bounce off of it, just like most of the light that reaches a mirror bounces off of it. But on the other hand, what if instead of having a very smooth surface, we made it very rough, like coarse sandpaper. Well, in that case, the reflectivity of this material would go down. And therefore, it would be more likely to absorb and then emit infrared radiation and reflect it.

Then, this third factor, object temperature, has more to do with emission than it has to do with absorption. The temperature of a particular material has very little to do with whether it will absorb or reflect infrared radiation incident on it. But it has a lot to do with how much that material will emit IR. This phenomenon is actually independent of whether infrared radiation is landing on this material or not. It’s simply a property of the material, where the hotter it is, the more it will emit radiation. And as a quick side note, any material at any temperature above absolute zero will emit some infrared radiation, even if it’s a very small amount. This is another reason why infrared radiation is so interesting.

With visible light, with light that our eyes can see, there are probably only a few sources that we encounter any one time, maybe the sun if we’re standing outside or, if we’re inside a building, the lights in our room. When it comes to visible light, most objects are not sources. They just reflect the light. But in contrast to this, every object we encounter is a source of infrared radiation even when these objects are very cold. Even when they’re below freezing, they’re still giving off some IR. And therefore, they’re a source. But like we said that emission does depend on temperature. And the hotter an object is, the more it gives off.

Now, there’s one last factor on our list that affects IR absorption and emission. And that is surface area, the surface area of our material. Imagine that we double the length of both sides of our material, so that its overall area goes up by a factor of four. Well, in that case, our much larger material is capable of radiating away or emitting much more IR. And because of its larger size, it’s a bigger target for incoming infrared radiation. This fact that the larger the surface area of a material, the better it’s able to emit infrared radiation and therefore give off heat is put to use in the cooling of electronic systems.

Perhaps you’ve seen a block of metal, with lots of very thin metal fins coming off the back of it. The point of all these fins, separated by very small air gaps, is to create lots of surface area for heat to escape from this overall piece of metal. The design is taking advantage of the fact that increased surface area means an increased capacity to radiate away heat. Let’s look now at an example exercise related to this topic of infrared radiation.

Which of the following properties of objects does not directly affect the amount of infrared radiation it emits and absorbs? A) Surface area, B) Color, C) Reflectiveness, D) Mass, E) Temperature.

To figure out the answer to this question, let’s imagine we have some piece of material here. And there’s infrared radiation shining on it. What we can do is experiment with each one of these factors to see which of them does not directly affect the amount of IR that this material emits and absorbs. We’ll start off with this first option, surface area. As we explore this choice, let’s imagine that our source of infrared radiation, the radiation that’s falling on our material, is not a point source. We’ll say it’s an extended source, like the sun so that infrared radiation, heat from the sun, is shining down all over, not just at a single point on the ground.

Now, we can see that because of its surface area, we can call this area 𝐴, our material is limited in the amount of radiation it can absorb. If it was bigger, if its surface area was greater, then more of this radiation could land on it. And it could absorb more. And then, in addition to that, a greater surface area would mean this material is better at radiating away or emitting IR. So this factor, surface area, affects both the heating rate, how much radiation it absorbs, as well as its cooling rate, how much it emits. Since our question is asking about a property that does not directly affect emission and absorption, we know that this one isn’t our answer. Surface area does affect these things.

This brings us to our next option, which is color. Now, if our material is a light color, say that it’s white, then it’ll be highly likely to reflect any infrared radiation incident on it. Any radiation that’s reflected is of course not absorbed. So this property does affect absorption. On the other hand, if our material had a dark color, then it would be more likely to absorb radiation and less likely to reflect it. And we know that a strong infrared absorber is also a strong infrared emitter. So the color of a material, whether it’s dark or light, does indeed affect the emission and absorption of infrared radiation. Therefore, option B isn’t our choice either.

Our next choice, reflectiveness, has to do with how likely this material is to reflect infrared radiation incident on it. If the material is very smooth and polished like a mirror, it will have a high degree of reflectivity and therefore a low level of absorption. But then, the opposite can be true as well, that our material is very rough, which makes it better at absorbing infrared radiation and worse at reflecting it. We see then that this property of reflectiveness does directly affect emission and absorption.

Option D suggests that mass does not directly affect these properties. Well, let’s imagine a scenario, where we had a material with a given surface area, color, and reflectiveness. And as well as that, let’s say the material was fixed at a certain temperature. We’ll just call that temperature 𝑇. Now, if we could keep all of those four properties the same but change the mass of this material, then the question is would that affect the emission and absorption of infrared radiation? And the answer is that it would not, at least not directly. So option D, mass, looks like it may be our answer. But let’s check option E just to make sure.

If we were to vary the temperature of our material, say by heating it up, then the material would respond by emitting more infrared radiation, by giving it off. Since temperature does directly affect these properties, that’s not our answer, which means that it’s the mass of an object which does not directly affect the amount of IR it emits and absorbs.

Let’s take a moment now to summarize what we’ve learned about infrared radiation. First off, we saw that infrared radiation, often referred to as IR, is light that has a wavelength between 700 nanometres and one millimetre. That’s the place infrared radiation occupies on the electromagnetic spectrum. We learn further that infrared radiation is the kind of radiation responsible for the heat that we feel coming from hot objects. Furthermore, all objects, as long as their temperature is above absolute zero, emit infrared radiation. Finally, we saw that a material’s color, its surface area, its temperature, as well as its reflectivity are all properties that affect how it emits and absorbs infrared radiation.

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