![]() ![]() In order to jump to one of a limited number of allowed higher energy levels, the atom must gain a very specific amount of energy. In the atom’s ground state, the electrons are in their lowest energy states. The electrons in an atom may be in a number of allowed energy states. Why do atoms absorb only electromagnetic energy of a particular wavelength? And why do they emit only energy of these same wavelengths? What follows here is a summarized explanation, but for a more comprehensive one, see Kaufmann’s Universe, pages 90-96. Those same wavelengths appear in emission when the gas is observed at an angle with respect to the radiation source. As the radiation passes through a gas, certain wavelengths are absorbed. ![]() The same phenomena are at work in the non-visible portions of the spectrum, including the radio range. ![]() Pattern of bright spectral lines (called emission lines) is seen against an other-wise If the gas is viewed at an angle away from the source of the continuous spectrum, a (called absorption lines) appear in the continuous spectrum.ġ. When a continuous spectrum is viewed through some cool gas, dark spectral lines These phenomena are known as Kirchhoff’s laws of spectral analysis:ġ. The emission lines are at the exact frequencies of the absorption lines for a given gas. If we can observe this re-emitted energy with little or no back lighting (for example, when we look at clouds of gas in the space between the stars), we will see bright emission lines against a dark background. The atoms or molecules in the gas then re-emit energy at those same wavelengths. The radiation emerging from the gas cloud will thus be missing those specific wavelengths, producing a spectrum with dark absorption lines. The particular wavelengths of energy absorbed are unique to the type of atom or molecule. However, when the radiation passes through a gas, some of the electrons in the atoms and molecules of the gas absorb some of the energy passing through. As described in Chapter 3, a blackbody object emits radiation of all wavelengths. ![]()
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