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.
