When I was at school we did experiments with bunsen burners and strontium wire. Which produces a red flame. Thus:
http://www.unit5.org/christjs/flamet6_small1.jpg

When I venture into the Physics section it is usually to ask questions.
Here is another one. Please correct me if I am wrong in any of these points.


1. The vivid red colour is produced because heating the metal makes strontium ions.

2. In the ions are excited electrons. Their movement into higher quantum states releases energy in the form of photons, in this case in the red frequency.

3. Heating different metals will produce other colours. eg copper green light etc

4. White light is a mixture of all the colours


So the question. Why is the sun's light white?
It is the conversion of hydrogen into helium which produces the light.
I understand that it is not simple burning, as with the strontium wire, but it is a single process all the same.
So why would this produce light at all visible frequencies instead of just one?

When I was at school we did experiments with bunsen burners and strontium wire. Which produces a red flame. Thus:
http://www.unit5.org/christjs/flamet6_small1.jpg

When I venture into the Physics section it is usually to ask questions.
Here is another one. Please correct me if I am wrong in any of these points.


1. The vivid red colour is produced because heating the metal makes strontium ions.

2. In the ions are excited electrons. Their movement into higher quantum states releases energy in the form of photons, in this case in the red frequency.

3. Heating different metals will produce other colours. eg copper green light etc

4. White light is a mixture of all the colours


So the question. Why is the sun's light white?
It is the conversion of hydrogen into helium which produces the light.
I understand that it is not simple burning, as with the strontium wire, but it is a single process all the same.
So why would this produce light at all visible frequencies instead of just one?

It's because the sun's light comes not only from the nuclear reaction (there are actually several different ones going on besides H/He) and ALL the metals and nonmetals are present in the sun. And ALL those metals are heated and emit photons just like your wire in school.

As a result, the radiation from the sun is called "full spectrum" because it not only produces all the colors of visible light (which therefore appears white) by at frequencies FAR above and below that of just visible light.

Thanks Read only.
That is a surprisingly easy answer.
I had forgotten about the other elements being present in the Sun.

That's it then, unless someone disagrees with you,
and if I know sciforums they probably will.:)

Thanks Read only.
That is a surprisingly easy answer.
I had forgotten about the other elements being present in the Sun.

That's it then, unless someone disagrees with you,
and if I know sciforums they probably will.:)

You're welcome, Cap.

And yes, I'm sure they probably will, especially since I left them a good opening. ;)

And here's that opening to ponder in the meanwhile (which, incidentally, also figures into your question about the sun - I gave you a rather short but accurate answer):

Why is it that the tungsten in an ordinary incandescent bulb can put out white light? And what about steel (and other metals) that can be heated to white-hot temperatures? Heh-heh! :D

I'll throw a spanner into the pigeons...err.. you know what I mean.
It is also a personal perspective thing.
I am (so called) red-green colourblind yet can see irridescent green flames in a fire when everyone else is seeing yellow or red. This happens in the low embers.

http://en.wikipedia.org/wiki/Black_body_radiation

It's because the sun's light comes not only from the nuclear reaction (there are actually several different ones going on besides H/He) and ALL the metals and nonmetals are present in the sun. And ALL those metals are heated and emit photons just like your wire in school.
The Sun's light does not come directly from nuclear reactions. The nuclear reactions occur only in the Sun's core. A gamma ray that results from such a reaction travels but a short distance before being absorbed. The ion that absorbed the gamma ray soon emits one or more photons, but at different frequencies. The reemitted photon travels but a short distance before it too is absorbed (and reemitted). The radiation generated by fusion takes a random walk to get to the Sun's surface. At the Sun's surface, the radiation is nearly that of a black body.
http://en.wikipedia.org/wiki/Black_body_radiation
To be precise, the Sun is not a black body (it is very close to one, however). Those excitation frequencies such as the strontium line are two way streets. Strontium is much more likely to absorb a photon at the strontium line than a photon with some random frequency. Suppose a thermal photon near the peak of the black body radiation curve is also near one of those excitation frequencies - for example, a photon at the hydrogen alpha line. Hydrogen atoms have a strong propensity to absorb such photons.The Sun emits less light at the excitation frequencies, not more. Notice the absorption bands in the attached image. (Source=http://climate.gsfc.nasa.gov/~cahalan/Radiation/Images/SolarIrr2.gif)
http://climate.gsfc.nasa.gov/~cahalan/Radiation/Images/SolarIrr2.gif

DH
That's slightly more complicated, but I like the theory.
The radiation (sort of) randomises itself on its passage outwards.

But what about Read only's tungsten filament question?

But what about Read only's tungsten filament question?
Were that explanation correct the spectrum of the light from the Sun would be zero at most frequencies, with non-zero contributions only near various spectral lines. Instead, the spectrum of the light from the Sun is nearly that of a black body at 5780 Kelvin. The key difference between an ideal black body and the actual solar spectrum is that in the vicinity of spectral lines the spectrum makes marked dips.

Were that explanation correct the spectrum of the light from the Sun would be zero at most frequencies, with non-zero contributions only near various spectral lines. Instead, the spectrum of the light from the Sun is nearly that of a black body at 5780 Kelvin. The key difference between an ideal black body and the actual solar spectrum is that in the vicinity of spectral lines the spectrum makes marked dips.

Yes, the opening I left in my short explaination was Plank's equations for Black Body radiation. :) But what you - and everyone else - have so far not presented is exactly how photons all across the spectrum are generated at ultra-high temperatures. ;) And THAT'S what I was hinting at with my original answer to Captain K.

But what you - and everyone else - have so far not presented is exactly how photons all across the spectrum are generated at ultra-high temperatures. ;) And THAT'S what I was hinting at with my original answer to Captain K.
That is not the question Captain K asked. He asked why sunlight is "white". Black body radiation is a much, much better explanation of this phenomenon than spectral lines. Sunlight exists because of nuclear fusion deep in the Sun's core. Sunlight is "white" because it is very close to a black body radiator. Explaining the Sun's spectrum as a consequence of excitation frequencies is simply wrong. If excitation frequencies were the primary driver one would expect to see spectral peaks rather than troughs at excitation frequencies.

Odd that our brightest source of light should be so because it is effectively a black body

Read Only. I would bet that you knew of this other explanation.
Do you disagree with it, or do you believe that the presence of heavier atoms
causes/affects the randomisation?

Re the tungsten filament. That must also be becoming a black body I suppose.
There's no thickness, so my guess is that the photons become randomised travelling through the wire in a circle
before emerging.

Odd that our brightest source of light should be so because it is effectively a black body
That's just a name.

But what you - and everyone else - have so far not presented is exactly how photons all across the spectrum are generated at ultra-high temperatures. ;) And THAT'S what I was hinting at with my original answer to Captain K.
It's quite complex. I think the first attempt at explaining the blackbody radiation curve led to the ultraviolet catastrophe (http://en.wikipedia.org/wiki/Ultraviolet_catastrophe) which was only resolved by quantum mechanics.

But what about tungsten light filaments???
Why do they emit white light?

The electric current flowing through the tungsten filament makes the filament get very hot. The tungsten filament acts very much like a blackbody radiator.

I suppose that when we use instruments to measure the spectrum of sunlight light we could argue that the “relative balance” or completeness of spectral lines would let us set a standard of what “White light” is, but we are still perspective orientated.
As a scuba diver experiences sunlight at depth much of the low end is stripped off yet the divers perception is that the sunlight present is still “white”. Someone who has a red-green issue (diminished sensation of red) will define their own “white”.
This doesn’t address the original question but should separate perspective.

The tungsten filament acts very much like a blackbody radiator.

Why would heat make it emit light at all frequencies instead of just one?

The tungsten filament acts very much like a blackbody radiator.

Why would heat make it emit light at all frequencies instead of just one?

Because that's what a black body radiator does. The peak moves, but the spectrum of a black body radiator is continuous.

Didn't Planck show exactly otherwise - the spectrum of a blackbody radiator is discontinuous? Hence quantum theory?

Because that's what a black body radiator does. The peak moves, but the spectrum of a black body radiator is continuous.

But how would heat make a tungsten filament a black body radiator?

The strontium ions mentioned in the first post emit red light because of transitions in a single ion. Thermal radiation is a very different beast than the spectral line emission you see from the strontium ions or even the spectral band emission you see in the blue flame. Note well: the ions (and the flame) also emit thermal radiation. However, a 2000oC flame temperature is too low to emit visible light. The thermal radiation a 2000oC flame is well into the infrared range.

So what is thermal radiation? The tungsten atoms in the filament are bound to each other, forming a lattice. There is some separation between atoms in the lattice at which the potential energy reaches a minimum. At absolute zero, all of the atoms will be separated by this minimizing separation distance. At a nonzero temperature the atoms move around. The atoms can lose some of this energy by emitting photons. These thermal photons obey quite different rules than the spectral line photons you see in the flame from a bunsen burner.

In the case of spectral line emission, radiation is at a very specific frequency. In the case of thermal radiation, the radiation is continuous.

Didn't Planck show exactly otherwise - the spectrum of a blackbody radiator is discontinuous? Hence quantum theory?
Planck showed that the energy is quantized. A photon can still have an arbitrary frequency.

Thanks DH.
I think I get the idea now.

Energy absorbed is re-emitted as photons at a range of frequencies depending on the temperature of the heated body.