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Question Video: Infrared Radiation Emission and Absorption Physics

Infrared radiation is directed toward an object. The object absorbs the infrared radiation and then emits it. Which of the following diagrams best represents the behavior of the object? [A] Diagram A [B] Diagram B [C] Diagram C

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Video Transcript

Infrared radiation is directed toward an object. The object absorbs the infrared radiation and then emits it. Which of the following diagrams best represents the behavior of the object?

In these three different diagrams, we see three different representations for how this infrared radiation absorption and emission occurs. In all three, infrared radiation is incident on the surface from above. The difference between them comes down to how infrared radiation is then emitted. The key point for us to realize is that before infrared radiation can be emitted, it must first be absorbed.

Note that in option (A), the emitted radiation seems to be reflected off of the surface. The incoming and outgoing radiation follow a path we would expect for a reflected light ray. Infrared radiation is light, light that our eyes aren’t sensitive to. But as we mentioned, for infrared radiation to be emitted, it must first be absorbed. We can say then that option (A) is not an accurate reflection of this process.

In option (C), the radiation seems to pass right through the surface. When this happens to infrared radiation, we would say that that radiation is transmitted. If that happens, though, this infrared radiation here is not really being emitted. It’s not being taken in, we could say, to this object and then radiated out. If answer option (A) represented reflection, answer option (C) represents transmission. Neither of those processes involves absorption.

Answer option (B) best represents an object that first absorbs and then emits infrared radiation. Note that absorbing this infrared radiation will tend to increase the temperature of the object. We know from experience that a heated object will tend to radiate out in all directions, and we see that in this example. Our final answer is option (B).

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