Video Transcript
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.
