Question Video: Thermal Radiation Physics

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


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.

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