The Spectrum |
(Think about these carefully before you consult the answers below)
QUESTIONS
1) Considering what you may already know about the elements, which would you think would have a more complicated spectrum, helium or oxygen? <answer>
2) The emission spectrum of hydrogen has red, blue, violet and other colors. When mixed together, what color do you think will result? That is, if you had a tube of hydrogen gas that was heated (or electrically excited) to incandescence, what color would it glow? <answer>
3) Except for a few unusual stars, the spectra of all stars show absorption characteristics. That is, they are continuous spectra with dark absorption lines. How can this be if stars are made of incandescent gases? <answer>
4) If you heat up a fireplace poker, it starts a dull red. As it gets hotter and brighter, it eventually turns to a yellow color if you get it hot enough. Is this because the wavelength of the emitted photons changes? <answer>
5) The observed spectra of the planets are quite similar to the spectrum of the Sun. Yet we know that they planets are very different from the Sun or stars. Why is this? <answer>
ANSWERS
1) The spectrum of oxygen is more complicated. Helium is a simple atom with only two electrons. Hence the number of permitted energy levels (and photons) is fairly limited. Oxygen has eight electrons and its permitted energy levels and possible photon energies are correspondingly much higher than those of helium.
2) Although the spectral lines from hydrogen are of several different colors, there combined light is not white but pinkish. This is because the strongest or brightest line in the spectrum is the bright red line known as the"Hydrogen Alpha" line (6563 Angstroms). It dominates the spectrum and when combined with the shorter wavelength lines gives a pink color.
3) Stars themselves produce continuous spectra because the region in which the photons originate is at high density because of immense gravitational forces. Incandescent solids, liquids and high density gases produce continuous spectra. But above this layer, a generally cooler and less dense region of gas exists. (In the case of the Sun this is called the <EM>Photosphere.</EM> It is in this cooler, more rarefied region that the atoms absorb some of the photons produced below. Because they absorb only specific photons corresponding to their permitted energy levels, an absorption spectrum results when viewed through a spectrograph.
4) No. The wavelengths of the photons themselves do not change. However, the temperature dictates the relative number of photons emitted at certain wavelengths. At relatively low temperatures, the redder photons are most in abundance. As it gets hotter, the greater number of photons emitted are of shorter, more yellow, wavelength. Hence the color change.
5) Planets do not undergo the same energy process as in the Sun and stars, and hence do not radiate much energy on their own. most of the light from a planet is simply reflected sunlight, so naturally the spectrum will be essentially the same as the Sun, only much dimmer. There are some slight differences due to the atmospheres of the planets.
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