Spectrum of a light bulb


by barryj
Tags: bulb, light, spectrum
barryj
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#1
Jan2-13, 10:36 PM
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I have been scanning a physics book. See attachment. The diagram shows that a light bulb has a continuous spectrum while the hydrogen has discrete spectral lines. Since the filament in the bulb is tungsten, why doesn't the light bulb also have discrete spectral lines?
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Drakkith
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#2
Jan2-13, 10:59 PM
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Solids, liquids, and very high pressure gases have a continuous spectrum due to Blackbody Radiation. Lower pressure gases such as the ones in lights have enough time between collisions in the gas to emit their extra energy in the form of radiation of a specific frequency. This occurs when the electrons drop energy levels and emit radiation with the exact energy of the difference between the energy levels. In solids and the other materials the energy levels are far more complex since there are many different bonds between atoms, and they can twist around, vibrate, and do a number of other things that end up emitting different frequencies of radiation. The end result is a continuous band instead of distinct frequencies. Very high pressure gases don't have enough time for their electrons to drop energy levels before they end up colliding and emitting radiation in a broad band as well I believe.
barryj
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Jan3-13, 08:20 AM
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Hmmm..... For me, it is a big jump to believe that black body radiation is continuous while ionized gasses emit quantized radiation. Sodium for example has discrete spectrum lines when in a sodium vapor light (I think). Would a solid piece of sodium not have discrete lines and exhibit a continuous spectrum? For discrete lines, does the material have to be vaporized?

AlephZero
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Jan3-13, 10:13 AM
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Spectrum of a light bulb


As Drakkith said, there are two different effects here.

The discrete spectral lines are caused by single atoms changing their internal energy states, for example an electron "jumping" from one orbit to another, if I can use that oversimplified description without getting torn apart on by quantum mechanics experts!

Black body radiation is caused by atoms or molecules interacting with each other - think of it more like the collisions between a collection of balls being shaken about in a box, where there is no restrictions on the amount of energy that can be transferred from one ball to another when they collide. (Again, that is over-simplified, but it gives the right general idea)

Both effects can occur at the same time. For example the light spectrum from the sun is mostly continuous, and the discrete lines are darker than the continuous level, not brighter. http://en.wikipedia.org/wiki/Fraunhofer_lines
barryj
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Jan3-13, 10:50 AM
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However, consider that the composition of stars is partially determined by looking at thee spectral lines. If a light bulb had a tungsten filament, then all the light should come from the tungsten being excited. If so, then there should be the spectral lines of tungsten, yes? I thought all light is due to electrons moving from one energy level to another, I guess not.
ModusPwnd
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Jan3-13, 10:57 AM
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Quote Quote by barryj View Post
I thought all light is due to electrons moving from one energy level to another, I guess not.
Why do you guess not?

It is do to energy levels. The energy levels in a solid are not the same as the energy levels in a gas. In a gas you are pretty much stuck with transitions of an electron in an atom but in a solid you have a whole host of interactions and possible transitions going on. So many that as your solid becomes bigger and bigger the individual lines representing possible transitions become so numerous that they look like a continuous band.
Drakkith
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Jan3-13, 11:01 AM
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No, not all light is the result of electronic transitions. And solid materials have bonds between the atoms which have different energy levels than single atoms do. So electronic transitions that do occur for these states have different energies than single atoms. in something like solid tungsten there can be a near continuous band of energy levels for the electrons instead of discrete ones.
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Jan3-13, 11:03 AM
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Quote Quote by barryj View Post
However, consider that the composition of stars is partially determined by looking at thee spectral lines. If a light bulb had a tungsten filament, then all the light should come from the tungsten being excited. If so, then there should be the spectral lines of tungsten, yes? I thought all light is due to electrons moving from one energy level to another, I guess not.
Starlight has bands of emission and absorption that are able to be seen in addition to its continuous spectrum. We can see them just because they are slightly darker or brighter bands within the spectrum.
barryj
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#9
Jan3-13, 12:13 PM
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Maybe this issue is due to the fact that quantum physics and "classical" physics do not always exactly converge. I see I have to do some more learning.

Thanks all.


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