 |
 |
 |
 |
 |
|
|
|
|
|
|
|
|
|
|
 |
 |
 |
 |
|
Generally, the light we see is composed of a mixture of wavelengths.
White light is composed of red, green, yellow, and blue photons. We
can separate light into its component wavelengths by using a dispersing
element, either a prism or a diffraction grating. Once the light is
dispersed, it forms a spectrum. A rainbow is an example of a spectrum
(the dispersing elements are water droplets which act as prisms).
The plural of "spectrum" is "spectra".
There are 2 basic kinds of spectra:
Spectra are a powerful tool for studying astronomical objects. A diffraction grating consists usually of thousands of narrow, closely spaced parallel slits (or grooves). These grooves selectively filter the light, spreading it into patches of specific wavelengths, so that the resulting image beyond the grating contains the color bars representing the signature pattern of the source of light. Elements which are gaseous can be identified by shining a light through them. As long as the light source has a continuous spectrum, and the gas is cooler than the source, then the image created by the diffraction grating identifies the gas by its absorption lines. Elements that are liquid or solid can be identified by examining the reflected light from a source with a continuous spectrum.
|
|
|
If you look at a dense gas, the photons you see have bounced around so much that you see radiation at a continuous range of wavelengths, often close to a blackbody spectrum. But if you look at a thin, transparent gas, then you can see photons
that come to you directly from an emitting atom. 1. A hot opaque body emits a continuous spectrum. That's what you get with light from the sun.
But when scientists look at the light coming off of just one element, hydrogen for instance, they don't see the whole rainbow. Instead they only see bright lines of certain colors. (Actually, "color" isn't the right term, because only some of the lines are visible, but for now we'll just talk about visible light.) The atoms of any individual element emit waves of very specific frequencies, unique to that element. When the spectrum of each element is viewed here on Earth, it is used as the identifying 'signature' of that element when those same spectral lines are seen from a cosmic body.
|
|
|
2. A hot gas emits photons with the characteristic wavelengths corresponding to the transitions between different energy levels of the atoms or molecules in the gas. This leads to bright lines, called emission lines, in the spectrum. If you put light from a common streetlamp through a prism, or look at the light through a diffraction grating, you will see distinct lines. Two common kinds of street lights use sodium vapor and mercury vapor bulbs. Each of these lights has a different spectral "signature", and you can tell what kind of lamp it is by its spectral lines. That is why different street lights seem to be different colors! This technique is so reliable that scientists can tell what elements they are looking at just by reading the lines. Spectroscopy is the science of using spectral lines to figure out what something is made of. That's how we know the composition of distant stars!
|
|
|
3. If you look at a hot source that emits continuous radiation and there is a cool gas between you and that source, then the cool gas absorbs photons with the characteristic wavelengths corresponding to the transitions between different energy levels of the atoms or molecules in the gas. This leads to dark lines, called absorption lines, in the spectrum.
When absorption lines are seen through a continuous spectrum source, they are used to identify the component elements in the cool gas; they are essentially the negative of that gas's emission lines. These emission and absorption lines for each element occur at unique frequencies. For instance, one of the radiation frequencies that Hydrogen emits is at 304 angstroms. Just like when you wear green sunglasses that only let in green light, when scientists put a filter on their telescope to only let in frequency 304 Å, they know they are looking at hydrogen, because that is the frequency that hydrogen emits here on Earth. On the 'Sun Filters' page, you can see solar images in twelve different frequencies. To learn more about the spectral signature of each element in the periodic table, load the Java applet at this University of Oregon site, click on any element, and choose emission or absorption lines. With the Java applet at this link
The spectra of molecules do get more complicated, but, to be sure, by knowing the spectra of a molecular structure on Earth, scientists can identify them when seen from a cosmic source. Diffraction gratings are used to examine EMR in the wavelength range of visible light. Other devices are used to do the same function in the other wavelength bands. |
|
|
That's how we know what the objects in the universe are made of! Ready for Doppler? Huh? What's that? |
|
