Absorption and Emission Spectrum


by physics kiddy
Tags: absorption, emission, spectrum
physics kiddy
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#1
Jul7-12, 04:52 AM
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When an element is excited by some method, it emits electromagnetic radiations of definite wavelengths. The arrangement of these wavelengths in order of increasing wavelength is called emission spectrum of the element. (as per my book)

But, the definition of absorption spectrum, I don't understand.It goes like this:

When a beam of continuous light is passed through a tube containing vapors or solution of the substance and the transmitted light is analysed with the help of a spectrometer , it is observed that the spectrum obtained contains a number of dark lines in otherwise continous spectrum. These dark lines appear due to the absorption of radiations of corresponding wavelengths by the substance. The dark lines in the absorption spectrum of a substance appear at the same position as the bright lines in the emission spectrum of the substance.

What does the definition mean. Specially, explain the underlined sentences. Thanks in advance for help.
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sophiecentaur
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#2
Jul7-12, 07:10 AM
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Presumably you have googled as well as posting this question here. This link includes a good diagram to show the distinction between emission and absorption spectra for a gas. You can also get the same effect when light passes through a transparent solution. (The spectral details are different, of course but the principle is the same).

The light that hasn't been absorbed travels straight through but the absorbed light is re-radiated in all directions, producing a gap (dip) in the transmitted spectrum at those wavelengths.
Redbelly98
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#3
Jul7-12, 08:14 AM
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The only underlined words I see are solutions of the substance, which would mean that the substance is dissolved in a liquid (typically water).

sophiecentaur
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#4
Jul7-12, 08:31 AM
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Absorption and Emission Spectrum


But I didn't see any point in just saying "it's the same". I couldn't actually see why the solution factor would make things different.
Let's wait for a response?
Redbelly98
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#5
Jul7-12, 09:15 AM
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I just thought physics kiddy did not understand what was meant by a solution of the substance. But yes, at this point they can ask if they want further clarification.
physics kiddy
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#6
Jul7-12, 10:10 AM
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Thank you sophi very much. The site was excellent and it helped me clear my doubts.
But I didn't get this line in the Black Body section:

Brick, iron or a dense gas will emit the same spectrum as long as they are at the same temperature. That spectrum will have a peak that lies at a particular wavelength, lambdamax.

Where did you get such a nice website ?
sophiecentaur
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Jul7-12, 10:46 AM
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@PK. First site I saw on google. ;-)

The spectra are the same when it's thermal (black body) radiation from a solid. For isolated atoms the spectra have characteristic lines. Pauli exclusion causes lines to spread into bands for more condensed matter. For solutions, I think this would be true also, to some extent.
rakeshkrmitra
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#8
Jul8-12, 07:47 AM
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absorption spectrum gives you the account of the radiation absorbed. if the energy is absorbed in a frequency region then that region appears dark in the spectrum and that is energy absorbed by the atoms or the constituent particles of the substance
physics kiddy
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#9
Jul13-12, 06:51 AM
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What does Heisingberg's Uncertainity Principle mean by saying delta x * delta v >=h/2pi ? x = position and v = velocity, right ? however their product doesn't mean anything.
CAF123
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#10
Jul13-12, 07:17 AM
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Heisenberg's Uncertainty Principle is an underlying quantum mechanical expression that says if you measure the position and velocity of a quantum particle, there is always a degree of uncertainty in your measurements. This uncertainty is nothing to do with our experimental technologies, it is derived from within quantum mechanics and associated mathematics.

The h bar over 2 is a constant, so the product of uncertainties in position and velocity have to be greater than this constant. With the best ever technology, we would have the product equal to h bar over 2.
Claude Bile
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#11
Jul13-12, 09:27 AM
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Suspension in this context just means that the molecule in question is suspended in a liquid as opposed to a gas. The absorption/emission spectrum will be largely unaffected by the environment for a macroscopic particle.

Claude.

P.S. Heisenbergs uncertainty principle doesn't have much to do with your original question. Try not to convolute unrelated topics.


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