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conantee
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« on: January 07, 2010, 09:46:05 PM »

83. A spectral line is produced by a gas that is sufficiently dense that the mean time between atomic collisions is much shorter than the mean lives of the atomic states reponsible for the line. Compared with the same line produced by a low-density gas, the line produced by the higher-density gas will appear
(A) the same
(B) more highly polarized
(C) broader
(D) shift toward the blue end of the spectrum
(E) split into a doublet
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« Reply #1 on: October 15, 2010, 11:23:03 AM »

This can be done by guess. What is the physical quantity discussed here? It is time. How is it different from case to case? For dense gas, time interval is short, but for low-density gas, time interval is long.

Let's eliminate some choices first. Since the density change is not discrete, (E) is unlikely to be true. There is no prefer direction in this problem, so (B) is also unlikely to be true. (A) is not famous choice - why bother with phenomena of no change?. Now, you can guess between (C) and (D).

Well, in quantum mechanics, we have notion of time, or more strictly, uncertainty of time, and uncertainty of energy in Heisenberg uncertainty principle. Hence, as \Delta t decreases in dense gas, \Delta E increases. That is, the uncertainty of frequency of light, and so wavelength, become more uncertain. The spectrum therefore is broaden.

The correct answer is (C). 45 of 100 people answered this question correctly.
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