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The Spectrum

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By passing sunlight through a glass prism, the great Sir Isaac Newton first proved that "white" light is composed of a variety of colors of light (p. 97).  Colors correspond to different wavelengths of light. As it passes through the prism, the wavelengths of light are bent . That is, they come out in a slightly different directions from the original beam as it entered the prism. The longer wavelengths of red light are bent less than the shorter wavelengths of blue light. Red, blue and other wavelengths of light combined together form white light. However, when passing through a prism, the differential bending of the light separates the colors to form the spectrum.

Different colors (wavelengths) added together make white light. Note that if you project a beam of red light onto a white wall in a dark room, you will get a red spot on the wall.

(Click on the image to change) If you then shine a beam of green light over part of the red light, the color where the two spots overlap will be yellow.

(Click on the image to change) If you then shine a blue beam on top of the other two, the area where all three overlap will be white. White light is sometimes referred to as the "presence of all colors." This is called "additive color." This is how colored wavelengths of light blend together, but interesting, just the opposite is true when you are talking about pigments or paints. For paints, red, green and blue will produce black!

The light from most objects, such as stars (which are dense gas at high temperature) or red hot fireplace pokers produce continuous spectra. Here is an example:

However, just because the spectrum is continuous, that doesn't mean that there will be "white" light. In the case of the fireplace poker, for instance, most of the light be be in the red wavelengths, and hence the hot poker will appear red.

Before long, scientists noted that some sources of light -- specifically heated, or electrically "excited" low density gases such as those in a fluorescent tube or neon sign -- do not produce a continuous spectrum. Instead, when spread out with a prism, these sources of light showed a limited number of very distinct wavelengths (colors). These show up as bright lines against a dark background, yielding what is called an "emission" spectrum. See image below for the emission spectrum of hydrogen:


To make things even more interesting, scientists also learned that if you pass the light from a continuous spectrum source through a low density gas, then some of the wavelengths will be removed, and dark lines will take their places in the otherwise continuous spectrum. What was most telling was that the dark lines in this "absorption" spectrum corresponded exactly to the bright lines in the emission spectrum if the same gas was used in each case. For instance, below is the absorption spectrum of hydrogen:


Note that the positions of the dark lines in this spectrum correspond exactly to the positions of the bright lines in the spectrum before it.

So just what is going on? Why aren't all light sources continuous?
Early in the 20th Century, physicists discovered that light is not produced in continuous waves as previously through, but rather as streams of light particles called photons . Each photon has a specific wavelength and a very specific energy quantity. They are said to be quanticized and are sometimes called light quanta. Photons are produced or absorbed when electrons move between energy levels in atoms. Since the electrons can exist at only very specific levels in the atoms, specific atoms produce only very specific photons. Thus all hydrogen atoms throughout the Universe will produce exactly the same type of photon and produce only certain wavelengths of light.

(Click on the image to the right to change)
Click A: Imagine an atom in an uncompressed or low density gas. In such a situation, the atoms are relatively far apart, and their energy levels or shells do not overlap.

  Click B: Electrons "prefer" to be in their lowest energy level. When an electron jumps down from one level to another, it emits a photon whose energy exactly matches the difference in energy levels. In an uncompressed gas -- that is, one in which the atoms are relatively far apart -- this will result in an emission spectrum. The photon will have a specific energy level (usually signified by the Greek letter "Lambda"). Note that when the situation is reversed, and an electron absorbs energy from a photon, it can do so only if that photon's energy exactly matches the energy difference between two energy levels in the atom. Passing light from a continuous source through a cooler uncompressed gas with yield an absorption spectrum as electrons in the gas absorb photons with specific energy levels corresponding to the energy level differences in the atoms. In other words, atoms of specific elements can emit or absorb photons only at very specific wavelengths or colors.

  Click C: In an uncompressed or low density gas, only certain energy transitions are allowed. However, when the atoms are pressed close together, their energy levels begin to overlap. In this case -- that of a high density gas, a liquid or a solid -- virtually any energy transition is possible, and many different photons and wavelengths are produced, not just a limited few. A continuous spectrum results.

Since the energy transitions and photons are very specific for specific elements, the lines in a spectrum can be used to determine the elements involved. The spectral lines are much like fingerprints or DNA sequences. Keep in mind that this is only part of what the light from distant stars can tell us. Other techniques can yield density, rotation, motion toward or away from Earth, and many other factors.
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