PrepTest 115, Section 3, Question 19
With the approach of the twentieth century, the(454 more words not shown)
Which one of the following(15 more words not shown)
- ARadiation reflected by(13 more words not shown)
- BAny object in(9 more words not shown)
- CAll blackbody objects(16 more words not shown)
- DAny blackbody object(14 more words not shown)
- EThermal radiation cannot(5 more words not shown)
Reading the passage
The blackbody discussion is about measuring an object's thermal radiation. We use materials such as black velvet and soot, and we're told that physicists can then be confident about what they're observing. For this inference, I'll check what that setup supports rather than assume every plausible claim about experiments is true.
Working the answers
- ACorrect
At first, I hesitate: if physicists can confidently measure the object's own radiation, doesn't that suggest reflected and emitted radiation are easy to distinguish?
Go back one sentence. The experiment requires objects with “little or no reflective capability.” That's what makes the confidence possible. Removing reflection prevents radiation from elsewhere from being mixed up with the object's own emission. The special setup solves a difficulty; it doesn't show that distinguishing the sources would be easy without it. Reading the two sentences together resolves the apparent contradiction.
- B
We aren't told that putting any object in a dark room makes it a nearly ideal blackbody. The specified property is low reflectivity.
- C
Nothing gives us a rule that equally sized objects emit at the same wavelengths regardless of temperature. That comparison isn't made.
- D
Ease of changing an object's temperature is never given as a criterion for whether it would be useful in an experiment.
- E
Thermal radiation does come from blackbody objects. That's the radiation the experimenters are trying to measure.
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