Re: Light
- From: "Danny Rich" <DannyRich@xxxxxxxxxxxx>
- Date: Fri, 30 Dec 2005 07:53:25 -0500
Roger,
Mr. Russel's comments are generally on point but, as he suspected, he has
missed a few details.
While a traveling wave electro-magnetic wave does contain both an electric
and magnetic vector component I know of no way to measure them
independently. If you look at Maxwell's equations (only 4 of them) they
describe the propagation of electromagnetic energy. The magetic and
electric vectors are dependent. A changing electric field produces a
magnetic field and a changing magnetic field produces an electric field.
As Mr. Russel indicated light is only one form of EM energy. The only
difference between light and other forms of EM energy is the frequency and
the frequency difference implies a difference in instrinsic energy from
Plank's law (E=hf). The wavelength is inversely proportional to the
wavelength so that low energy waves have long wavelengths and high energy
waves have short wavelength.
The emission of radiation is an energy driven event. The emission of EM
energy does not have to be driven by electron orbitals. Gamma rays are the
result of nuclear processes and electrons are not involved. Other sources
of light are also possible based the vibrations in crystal lattices, and the
confromation of atoms in a solid. Kurt Nassau's book on 15 causes of color
may help here.
The doppler broadening of an emission can be due more to motion of the atoms
in the gas or plasma than to motions in the electrons. There are no clearly
identifiable emission bands in sunlight - even in extra terrestrial
sunlight. But there are absorption bands due to the light passing through
the gasses in the "cooler" outer layers. As the wave passes by or through
the gas molecule the energy of the light may be equal to the resonance
energy of the either the electron or atomic configuration of the gas
molecule. Then the energy of the wave is transferred to the energy of the
molecule and an excited state is produced.
The effect of light on an object is modulated by its energy and is this
quantized. Certain other behaviors of a light rays, as you seem to recall,
are best described by picturing the light beam as a stream of particles and
not as a continuous wave.
Feynman's books on introductory Physics are well known but are not as easy
to follow as their reputation seems to indicate. They were, after all,
written for his students at CalTech and those classes are definitely not
composed of mainstream North American technologists. Anecdotal comments
seem to indicate that the faculty enjoyed the lectures far more than the
students. Today, there are a large number of well written texts for the
non-physicist that do a much better job of explaining optical physics, such
as Light and Color in Open Air by Minnaert. Spend an evening in a large
public or university library and you will be amazed at what is currently in
print.
Danny Rich
"Roger Breton" <graxx@xxxxxxxxxxxx> wrote in message
news:BFD9FC1B.14AD1%graxx@xxxxxxxxxxxxxxx
> Dear Mike,
>
> Thank's for your most helpful comments.
>
>> Light is composed of two oscillating fields oriented at 90 degrees to one
>> another, one magnetic and the other electrical. These may be measured
>> using
>> magnetic or electrical detectors.
>
> OK. Those two fields are normal from each other and propagate in the same
> direction at the same speed. But how would those two fields be different,
> physically, from, say, a radio wave or a television wave or a gamma wave:
> is
> it just the length or frequency? Same two basic components, electric and
> magnetic?
>
> Then, if that's the case, if the only differentiating factors is the
> frequency, radio or tv waves measure in the kilometer range, I gather, can
> I
> safely say that only "light" waves carry photons? I wonder now what is it
> that TV or radio or radar or other waves carry, if anything at all? We
> speak
> of the "carrier" in the context of telephone, like 440 Khz tones but all
> these are, and I guess all sound waves by extension is "pure" frequencies?
> Strictly electric fields oscillations, no? Whereas what defines a "light"
> wave (i.e.appearing in the visible part of the electromagnetic spectrum)
> is
> the fact that is carries photons? But does not UV also carries photons?
> I'm
> not sure about cosmic rays or infrared. Boy, I'm sorry I opened up that
> can
> of worms...but I could not resist. If you have some explanations to offer
> I'll gladly take them, Mike.
>
>> Likewise, the size of the waves determines the wavelength.
>
> Yes, but what is it that determines the size of the the wave? (chicken and
> the egg question?)
>
>> The continuous spectrum of the sun, and any very hot object, is due to
>> thermal doppler shift of discretely emitted wavelengths.
>
> I remember something to the effect of the umpteen spectral lines of the
> sun.
> Indeed, that always triggered the question in my mind as to, hey, what
> happens with the rest of the "visible spectrum" from 380nm to 730nm, more
> or
> less? You say, the whathever limited number of discrete spectral lines
> that
> make the underlying sun solar spectrum is what's reponsible for the
> "continuous" spectrum by thermal doppler shift? I'm not going to ask how
> that happens.
>
>> Here you are touching on the wave-particle duality of matter and light.
>> http://en.wikipedia.org/wiki/Wave-particle_duality
>
> Well, how could I ever forget my high shool physics class? If there is
> anything I retained from that era was the Louis DeBroglie 1924 wave
> particle
> duality (Or was it in college chemistry when we learnt the Shroëdinger
> equation?)
>
>> No. The rate of re-excitiation determines the flux, or number of photons
>> per unit of time.
>
> OK. But the rate of re-excitation must correspond to some discrete events,
> no?
>
>> The quanta is equal to the difference in energy level of
>> the electron,
>
> So, the higher the difference in energy level, the higher the quanta of
> light? Are there larger quanta than others? Does this corresponds to
> discrete levels of electron excitation orbitals like SP2, DSPx?
>
>> and determines the frequency, which is equal to the energy of
>> the emitted photon.
>
> So light is the result of energy level drops, which determine the
> frequency
> of the emitted photon? OK. Let's say I dial in 550nm on my monochromator.
> I
> shine some 35W quartz halogen lamp in its 1.34mm entrance slit. When I
> observe the appearance of the "light" coming through at the exit slit,
> what
> can I say, physically, about the process of producing that 550nm
> radiation?
> Can I say it is the result of selecting a very narrow band out of the
> polychromatic illumination that we know was produced by the tungsten
> filament? Can I say further that the tungsten filament is made to emit
> light, photons, in a 380nm to 730nm range by flowing a 12V current through
> it at a constant rate? Now, what is it, in yor opinion, in that tungsten
> filament that causes it to emit visible energy in a continuous spectrum
> fashion? It is the nature of the tungsten atoms themselves that reach a
> series of discrete levels of energy, one for each visible wavelenght?
> Sounds
> like a little far-fetch account for what is going on physically there. But
> that's the best "theory" I can offer today :(
>
> In constrast, it's easy to track down what happens with gases like mercury
> and oxygen and xenon or neon because all those, by definition emit at a
> limited number of discrete wavelengths.
>
>> This will be an interesting class, as you are obviously capable of
>> re-examining questions from a fresh viewpoint.
>
> Hope I don't end up boring my students too much...
>
> Roger Breton
>
.
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