Question Video: Comparing Infrared Radiation Absorption and Emission across Objects Physics

The table shows information about two different objects. Object 2 absorbs more infrared radiation than object 1. Which of the following, if any, is not a reason for this? [A] Object 2 is black, whereas object 1 is white. [B] Object 2 has a low reflectivity, whereas object 1 has a high reflectivity. [C] The temperature of object 2 is higher than that of object 1. [D] The surface area of object 2 is greater than that of object 1. [E] The above statements are all reasons why object 2 absorbs more infrared radiation than object 1.

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Video Transcript

The following table shows information about two different objects. Object two absorbs more infrared radiation than object one. Which of the following, if any, is not a reason for this? (A) Object two is black, whereas object one is white. (B) Object two has a low reflectivity, whereas object one has a high reflectivity. (C) The temperature of object two is higher than that of object one. (D) The surface area of object two is greater than that of object one. (E) The above statements are all reasons why object two absorbs more infrared radiation than object one.

From our table, we see that we have these two objects, object one and object two, and that these objects differ according to their properties of color, reflectivity, temperature, and surface area. If we were to make sketches of these two objects, object one might look like this, while object two might look like this. We are told that of these two objects, object two absorbs more infrared radiation than object one. We want to identify which of these answer options, if any, is a reason why that is not the case, in other words, which of these options, if any, does not give a reason why object two absorbs more infrared radiation than object one.

One important thing to keep in mind is that we’re talking about absorption of infrared radiation rather than, say, emission. In terms of differences between these objects, we see that object one is white, while object two is black. Object one has a high reflectivity, while object two has a low reflectivity. The temperature of object one is 20 degrees Celsius, while the temperature of object two is twice that. And lastly, the surface area of object one is one-half the surface area of object two. Compared to object one then, object two is larger, it’s hotter, it’s darker, and it also reflects incoming radiation less.

Several of these differences do help object two absorb more infrared radiation than object one. For example, the fact that object two is bigger means that it has a larger surface area with which to absorb incoming radiation. This means that answer option (D) that the surface area of object two is greater than that of object one really is a reason why object two can absorb more infrared radiation than object one. Recall that we’re looking for an explanation that is not a reason for this. Since answer option (D) is a legitimate reason, we won’t choose that as our answer.

Answer option (A), which notes that object two is black while object one is white, also names another reason why object two would tend to absorb more infrared radiation than object one. Even if these two objects otherwise had the same reflectivity, the same temperature, and the same surface area, if one of the objects has a darker color, then that object will tend to absorb more infrared radiation. Since option (A) gives a real reason for the difference in absorption, we won’t choose that as our answer either.

Answer option (B) points out the difference in reflectivity between these objects, that the reflectivity of object two is lower relative to that of object one. This also would tend to make object two absorb more infrared radiation than object one. Answer option (B) is off our list.

Let’s look now at option (C). This choice notes that the temperature of object two is higher than that of object one. Looking back at our table, we see that this is true. The temperature of object two is twice that of object one. However, this difference in temperature doesn’t affect how the two objects absorb infrared radiation. If we were talking about emission of infrared radiation, then, all other things being equal, a hotter object will tend to emit more infrared. In terms of absorption though, temperature does not have an impact. Therefore, option (C) is not a reason why object two absorbs more infrared radiation than object one.

Therefore, answer option (E), which claims that all four of the above options are truly reasons why object two absorbs more infrared radiation than object one, can’t be correct. For our answer, we choose option (C): the temperature of object two being higher than that of object one is not a reason why object two absorbs more infrared radiation than object one.

Let’s look now at part two of our question.

Object two emits more infrared radiation than object one. Which of the following, if any, is not a reason for this? (A) Object two is black, whereas object one is white. (B) Object two has a low reflectivity, whereas object one has a high reflectivity. (C) The temperature of object two is greater than that of object one. (D) The surface area of object two is greater than that of object one. And (E) the above statements are all reasons why object two emits more infrared radiation than object one.

If we again draw examples of what objects one and two might look like, we note that in this case, we’re talking about infrared radiation emission rather than absorption. Just as object two absorbs more infrared radiation than object one, so it emits more. In this part of our question, we want to identify which, if any, of these options is not a reason for this; that is, do any of these statements not explain why object two emits more infrared radiation than object one?

Starting with answer choice (A), the fact that object two is black whereas object one is white is a reason why object two would tend to emit more infrared radiation. If these objects were identical in all ways except a color difference, with object two being darker, for that reason alone, we would expect it to emit more infrared radiation than object one. Option (A) being a valid reason for this difference in emission between the two objects won’t be our answer choice.

Option (B) as we saw points out the difference in reflectivity between these objects. All other things being equal, an object with low reflectivity will tend to emit more infrared radiation than one with high reflectivity. This is legitimately a reason why object two would emit more infrared radiation than object one.

Choice (C) describes the temperature difference between these two objects. Since we are now talking about emission, this temperature difference does play a role. A hotter object will tend to emit more infrared radiation than a cooler one. Option (C) then does help to explain why object two emits more infrared radiation than object one, and so does option (D) that object two is larger than object one. A greater surface area means that there are more opportunities for emission. Choice (D) won’t be our answer either.

Our findings agree with answer option (E). The above statements are all reasons why object two emits more infrared radiation than object one. For our answer, we choose option (E).

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