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Q & A: Emission and absorption spectra

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Most recent answer: 10/22/2007
Q:
1.what is emmision spectrum and absoption spectrum? 2.application of spectrum
- ben
yu yuan secondary school, sandakan,sabah,malaysia
A:
Hi Ben,

The emission spectrum of a material is the set of frequencies (and relative intensities of light emitted at those frequencies) of light given off by the material. Some materials give off light of only a few specific frequencies when they are excited in specific ways. For example, if an electric current is passed through a vapor of mecury atoms, only the frequencies which correspond to transitions between various configurations of the electrons around the orbit of the mercury atoms appear. Sodium atoms under similar circumstances give off a different spectrum, which is why their light is yellow and mercury vapor lights are more whitish (actually they're a little green). Neon gives off a nice orangey light. Hydrogen is the simplest case to calculate from first principles, since it has only one electron and one proton. It is the emission spectrum of hydrogen which gave physicists in the early 20th century some of the most important clues about the quantum mechanical behavior of electrons in atoms.

Most objects consist of large collections of atoms stuck together in some way. In such large collections, there are many possible energy states, not just the few specific ones that are allowed to the electrons in single atoms all by themselves. If a large collection of atoms is heated up, the spectrum of light it emits is continuous, and is called the "blackbody radiation law." A real spectrum may have both kinds of components -- blackbody and discrete emission by atoms. By measuring how much light is at each frequency, the temperature of the emitting object may be deduced (this is often how the temperature of glowing molten steel in steel mills is ascertained -- thermometers would melt).

Absorption is just emission in reverse. Atoms can absorb light at the same frequencies they emit at, going from their lowest-energy state to an excited state. If they re-emit right away at the same frequency, it's called scattering. If they emit light at some other frequency, it's called fluoresence.

The sun is very hot and emits lots of light at all frequencies, mostly following the blackbody curve. If some cooler material is at the surface of the sun or above it isn't ionized (that is, the atoms still have their electrons orbiting their nuclei), then these atoms may absorb some of the light emitted by the sun, but only at the frequencies which correspond to electronic transitions. Early observers of the sun used prisms and diffraction gratings to find narrow bands of absorption in the solar spectrum, and lined these up with the similar bands observed in the laboratory, and thus deduced the chemical composition of the outer layers of the sun. Similar investigations have shown us much about the chemical composition of faraway stars and interstellar gas and dust. Doppler shifting of these spectral lines also tells us about the speed of astronomical objects along the direction of sight.

Tom

(published on 10/22/2007)

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