大学物理-量子物理 (1)

Transcript

05-1-Intro to QM

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101 AvatarIntro

CHAUCER: Do you two know how scientists identify the substances in distant stars? Do you know how we measure the composition of the sun for example? KEVIN: Sure, one major tool is spectral analysis isn’t it?

CHAUCER: Yes indeed.

DIANA: I am familiar with the results of spectral analysis, but I am a bit unsure of how the spectra arise to begin with. Chaucer, is that part of our material for today?

CHAUCER: Yes it is Diana, in fact it is the starting point. Let’s use Professor Peabody’s WABAC Machine, since we don’t have one of our own, and look at a little science history. Jeeves?

102 Spectra

JEEVES:

Traveling to the 1750’s we find that scientists were putting different substances in flames and passing the resultant light through a prism. They found that the hot gases given off by the burning materials emitted different colors of light or spectra.. Ordinary table salt, for example, generated a "bright yellow" spectra.

Furthermore, not all the colors of the rainbow appeared - there were dark gaps in the spectrum, in fact for some materials there were just a few patches of light. By the 1820's, they recognized that spectra provided an excellent way to detect and identify small quantities of an element in a powder put into a flame.

Meanwhile, the white light of the sun was also being examined closely. And in 1802 it was discovered that the solar spectrum itself had tiny gaps - there were many thin dark lines in the rainbow of colors.

But the reason for the spectral lines in the light and the relationship to each substance was a real mystery.

105 BlackBody Radiation

Traveling forward from that era to a little over 100 years ago, scientists were examining the colors of light given off by solid heated objects, they discovered that these hot solids gave off continuous spectra and that the overall color of the light revealed the temperature of the object. Now this was important because scientists realized that this discovery made it possible to measure the temperature of an object from a distance…they could measure the temperature of the sun for instance!

During these discoveries, they also noticed that some objects absorbed light extremely well … almost perfectly in fact.

They were called Black Bodies because they absorbed virtually all of the light that shone upon their surfaces. These same objects also radiated almost perfectly, and as noted before, the temperature of the black body object determined the distribution of colors or wavelengths in the emitted light.

This curve shows how much light of each color is emitted by a cool object. As you can see there isn’t much light and what light there is mostly lies out past the red end of the spectrum in the infrared. The figure on the right will show the different colors added together as we progress. Right now only red light is visible.

This curve shows how much light of each color is emitted by a medium temperature object. And the most light is emitted in the orange-yellow-green wavelengths. So we will add orange, yellow and green to our cauldron of light on the right. As you can see the combination so far looks yellow.

This third curve is for a really hot object – lots of light and with most of it being emitted toward the blue end of the spectrum. And as you can see our cauldron now is pure white in the center where all the colors overlap.

A heated black body follows this color path as the temperature rises.