dolimitless
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How is a continuous spectrum formed?
I know that solids, liquids and dense gases glow at high temperatures and produce a continous specutrum. I don't get how continuous spectrum's are
produced as opposed to an emission spectrum? I know in an emission spectrum the light/photons emitted at their respective frequencies give distinct
spectral lines, because of the elctrons dropping from an excited state back to their ground state, but how are continous spectrum's produced?
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dolimitless
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Is it because a white light moves at the speed of light, having a very fast frequency, is causes it to have all the possible vales for spectral lines?
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watson.fawkes
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Very (over-) simply,
it's Doppler shift. The thermal motion of the atoms spreads any spectrum out, broadening it, until the point where any discrete behavior fuses into a
continuously whole. Atoms move in random directions with respect to any fixed observer, and the observed frequency of emission changes accordingly.
For a full answer, you'll need to learn some significant amount of statistical mechanics.
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dolimitless
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Thanks, I am just looking for a general, grossly simplied explanation for a general chem I, chem II student. Can anybody enlighten me?
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dolimitless
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I found this explanation, is it valid?
When individual atoms, acting separately, emit photons, you get emission lines, because the photon energies correspond to differences in energy levels
within the atoms.
When molecules, acting separately, emit photons, you get emission lines, but many more of them, because molecules are more complicated than atoms, and
so they have a greater number of different energy levels.
When large collections of molecules are stuck together in a solid or liquid, you do not get emission lines. Really, there are emission lines, but
because there are no so very many energy states, there are so many (I mean REALLY a lot, like trillions) that they overlap and form a continuous
bright spectrum.
Therefore, hot gases and plasmas produce lines spectra, hot liquids and solids do not. The sun is technically a plasma ball, but it is so dense that
the ions cannot act independently, and so it acts like a liquid. (The sun is denser than water, after all, so it makes sense that the ions are jammed
pretty close together when you think about it.)
The sun and the incandescent bulb will produce continuous spectra, and the mercury vapor lamp and the candle will produce emission lines. The mercury
vapor lamp will produce the fewest and clearest lines. The chemistry in a candle flame is more complicated, and will produce a "messy" line spectrum,
so if you must choose only one answer, then the mercury vapor lamp is the best one.
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dolimitless
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Found another explanation:
Atoms are packed together so tightly that their outer electrons are influenced by neighbor atoms. Orbit separations which determine how the electron
can jump can no longer follow definite laws. With no definite orbits, an atom is no longer confined to radiating a definite set of wavelengths. It can
radiate any one of a variety of wavelengths because a variety of orbits are possible. At any given moment, billions of atoms in a solid are emitting
billions of different wavelengths. Hence the solid, liquid, or high pressure gas radiates a continuous spectrum.
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dolimitless
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Can anybody tell me why white light has a continuous spectrum as opposed to sunlight which has an absorbtion spectrum?
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not_important
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Could it be because both the Earth and the Sun have atmospheres ( http://en.wikipedia.org/wiki/Sun#Atmosphere ) and so light passing through them interacts with those atmospheres?
http://csep10.phys.utk.edu/astr162/lect/sun/spectrum.html
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JohnWW
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A "continuous spectrum" would be one due to thermal emission of radiation (primarily infrared, visible, and ultraviolet) which occurs, as per the
Planck Equation, for all ordinary matter above absolute zero. It depends on absolute temperature, for the intensities and the wavelength ranges of
radiation emitted. It would be different from an emission spectrum at highly specific frequencies due to electronic transitions between energy states
in atoms of elements, which is used as the basis for arc emission spectroscopic analysis of conducting materials (usually metallic alloys).
The Planck equation describes, for a perfectly black body, the intensities at various frequencies emitted, and the wavelength of peak emission at a
given temperature can be derived from it. Also derived from it is the Stefan-Boltzmann equation, according to which the intensity of emission is
proportional to the 4th power of absolute temperature. See Perrys Chemical Engineers Handbook, chapter 10, and good textbooks on heat transfer and
quantum thermodynamics.
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dolimitless
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I don't get why a cool gas when it absorbs energy (an absorbtion spectrum), why doesn't it re-emit it and cancel out the absorbtion?
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not_important
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Think about it - the gas is between you and the light source, which is radiating photons toward you. The gas absorbs photons at some wavelength, then
emits them in random directions. So you go from effectively a beam headed at you, to a full sphere, with only a small fraction of the light headed
your way.
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dolimitless
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Why does the gas emit the photons in random directions?
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12AX7
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How could it not?
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dolimitless
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I don't know that's why I am asking. Is that just a propety of gases? random movement?
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DJF90
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You can't expect the gas to decide it wants to fire the photons your way. It does so isotropically, much like a light bulb (over simplified I know...)
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Nicodem
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It would be absolutely one of the weirdest phenomenons if photons would be emitted in some special directions without any external influence.
The electron releasing its kinetic energy in the form of electromagnetic wave does not see "directions".
There is however a form of photon emission that is induced and as such it has not only the timing (same phase) but also direction. It takes the
direction of the passing by electromagnetic wave that induces the emission. Not just that, but it also inherit the same phase and energy (frequency).
Actually it is wrong to say that it inherits the energy identity since the electron transition energy must be of identical energy for the induction to
occur at all. This phenomenon can be forced to occur in a continuous fashion (new photons inducing more new photons emissions) and as such is called
"Light Amplification by Stimulated Emission of Radiation".
That is the only case of photons taking directions that I can think of.
PS: Please open pedagogical threads in the Beginnings section where I'm moving this. We have forum sections so that you can use them, so open threads
in the sections corresponding to the thread topic.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Nicodem
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Thread Moved
21-6-2009 at 07:09 |
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