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Thread: Light, Photons and Electromagnetic Spectra?

  1. #1 Light, Photons and Electromagnetic Spectra? 
    Your Mama! GiantEvil's Avatar
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    It is my understanding that the energy a photon may carry is quantized into Plancke units?
    As such a photon might carry only a single Plancke of energy, but is there an upper bound on the energy a photon may carry?
    Are there EMF's above Gamma?


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    exchemist
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    Quote Originally Posted by GiantEvil View Post
    It is my understanding that the energy a photon may carry is quantized into Plancke units?
    As such a photon might carry only a single Plancke of energy, but is there an upper bound on the energy a photon may carry?
    Are there EMF's above Gamma?
    No it can carry any energy it likes, subject only to any restrictions as to the frequencies that its source can emit.

    The relationship is E=hν, in which E is energy, h is Planck's Constant and ν (that's the Greek "nu"), is frequency. The quantisation comes in when the emitting source is considered. Quite a lot of these can only emit certain frequencies, corresponding to quantised energy jumps in whatever it is that emits the photons. For example sodium street lamps. But some sources, for example so-called "black bodies", such as a filament light bulb, emit a continuum of frequencies, so with them there are photons emitted with any frequency you like, - albeit within a certain overall range, determined by how hot the source is.


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    Hmm. Is it not the case that Einstein showed that the allowable energies of a propagating photon are given by ?

    Did he not get a Noble for proving that this explains the apparent quantization of absorption in the photoelectric effect?

    HaveI misunderstood something important here?
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Guitarist View Post
    Hmm. Is it not the case that Einstein showed that the allowable energies of a propagating photon are given by ?

    Did he not get a Noble for proving that this explains the apparent quantization of absorption in the photoelectric effect?

    HaveI misunderstood something important here?
    You are correct. And I assume it is the presence of in that equation which has misled the OP.

    Did he not get a Noble ...
    A Duke?
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by GiantEvil View Post
    As such a photon might carry only a single Plancke of energy, but is there an upper bound on the energy a photon may carry?
    The energy of a single photon is proportional to its frequency. It is not constrained to be an integer multiple of Planck's constant.
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  7. #6  
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    Quote Originally Posted by GiantEvil View Post
    is there an upper bound on the energy a photon may carry?
    No. By moving towards a source of photons at a speed that is arbitrarily close to the speed of light, one encounters photons that have arbitrarily high energy.


    Quote Originally Posted by GiantEvil View Post
    Are there EMF's above Gamma?
    It's worth noting that gamma radiation is not defined by its frequency, but by its source, specifically nuclear or subatomic processes. Thus, it is possible that there is gamma radiation that is indistinguishable from radio waves, although it is not known if such radiation actually exists. However, it is known that gamma radiation can have frequencies that are lower than some x-rays (which in terms of the upper frequency limit, are also defined by its source, specifically atomic or molecular processes).
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  8. #7  
    exchemist
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    Quote Originally Posted by Guitarist View Post
    Hmm. Is it not the case that Einstein showed that the allowable energies of a propagating photon are given by ?

    Did he not get a Noble for proving that this explains the apparent quantization of absorption in the photoelectric effect?

    HaveI misunderstood something important here?
    Maybe I expressed myself badly. Let me have another go. What I meant is that photon energies are not restricted to multiples of some unit called the "Planck", as the OP was implying might be the case. If the frequency of the photons is not restricted by constraints in the emitting source, a full range of frequencies can in principle be emitted and the photons emitted will thus have a full and continuous range of energies. If however one considers radiation of a only a single fixed frequency (i.e. monochromatic radiation), then that radiation will be in fixed amounts, each of hν.

    Any better?
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    Quote Originally Posted by exchemist View Post
    photon energies are not restricted to multiples of some unit called the "Planck", as the OP was implying might be the case.
    Now you lost me entirely.

    I had been led to believe that the energy of polychromatic light was quantized, and that each quantum of energy obeyed the principle . And that these quanta (later) came to be called "photons".

    Surely a single quantum of energy can have only one energy?
    If the frequency of the photons is not restricted by constraints in the emitting source, a full range of frequencies can in principle be emitted and the photons emitted will thus have a full and continuous range of energies.
    Which runs counter to my understanding of quantum physics (which I confess is not deep).
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  10. #9  
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    Quote Originally Posted by Guitarist View Post
    Quote Originally Posted by exchemist View Post
    photon energies are not restricted to multiples of some unit called the "Planck", as the OP was implying might be the case.
    Now you lost me entirely.

    I had been led to believe that the energy of polychromatic light was quantized, and that each quantum of energy obeyed the principle . And that these quanta (later) came to be called "photons".

    Surely a single quantum of energy can have only one energy?
    If the frequency of the photons is not restricted by constraints in the emitting source, a full range of frequencies can in principle be emitted and the photons emitted will thus have a full and continuous range of energies.
    Which runs counter to my understanding of quantum physics (which I confess is not deep).
    We get a continuous spectrum from a black body, yes? So there are photons emitted with a continuous range of energies. Each one carries hv of energy, but since v is continuous, so is the range of emitted energies of the photons comprising the radiation.

    That's all I'm saying. The OP appeared to me to suggest that photons could only exist with a one, two, three, or more, "Planckes" of energy.
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    Quote Originally Posted by exchemist;607904[/quote
    We get a continuous spectrum from a black body, yes?
    Of radiation, yes. Of energy no.
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    Quote Originally Posted by Guitarist View Post
    Quote Originally Posted by exchemist;607904[/quote
    We get a continuous spectrum from a black body, yes?
    Of radiation, yes. Of energy no.
    A continuous spectrum of frequencies, which implies continuous spectrum of energies.
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    Quote Originally Posted by Guitarist View Post
    Quote Originally Posted by exchemist View Post
    photon energies are not restricted to multiples of some unit called the "Planck", as the OP was implying might be the case.
    Now you lost me entirely.

    I had been led to believe that the energy of polychromatic light was quantized, and that each quantum of energy obeyed the principle . And that these quanta (later) came to be called "photons".

    Surely a single quantum of energy can have only one energy?
    If the frequency of the photons is not restricted by constraints in the emitting source, a full range of frequencies can in principle be emitted and the photons emitted will thus have a full and continuous range of energies.
    Which runs counter to my understanding of quantum physics (which I confess is not deep).
    A single photon is a quantum for a given frequency of electromagnetic radiation. It is the smallest discrete unit of energy light of that frequency can have. Photons for different frequencies of light will have different energies. So, for example, since "white" light is made up of a mixture of different frequencies of light, a sample of white light will contain photons of varying energies.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
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    Quote Originally Posted by Guitarist View Post
    Surely a single quantum of energy can have only one energy?
    Yes. One quantum has a single wavelength and a corresponding single energy. But the "next most energetic" photon does not have to add one Planck unit of energy. A single photon can have any value of wavelength and energy; that value does not have to be an integer multiple of h.


    (There seems to be some talking at cross-purposes here which makes it hard to clarify / understand what is meant....)
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  15. #14  
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    Quote Originally Posted by Janus View Post
    So, for example, since "white" light is made up of a mixture of different frequencies of light, a sample of white light will contain photons of varying energies.
    And the range of possible photon energies is continuous.
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  16. #15  
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    Just want to thank everyone for their answers. Busy lately, hope to have a follow up question sometime.
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    Quote Originally Posted by Strange View Post
    And the range of possible photon energies is continuous.
    How can that be if the photon IS the quantum of electromagnetic radiation energy? Doesn't this imply that the energy spectrum is not continuous? Isn't this (more-or-less) the definition of a quantum id est a photon?
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    Quote Originally Posted by Guitarist View Post
    Quote Originally Posted by Strange View Post
    And the range of possible photon energies is continuous.
    How can that be if the photon IS the quantum of electromagnetic radiation energy? Doesn't this imply that the energy spectrum is not continuous? Isn't this (more-or-less) the definition of a quantum id est a photon?
    Electromagnetic radiation of a given wavelength is quantised into photons with the corresponding energy. That is what quantisation means in the context of photons (see the photoelectric effect for more details). But the allowed values of wavelength or energy are not quantised.
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    Strange has it right. The energy is quantised, such that a purely monochromatic (strictly single frequency ) pulse of light will have the energy , where is Planck's constant and is an integer, the number of photons. The quantisation is because of the , not the or the .

    It might be possible to add an extra level of quantisation to the equation by finding some fundamental unit of time, since frequency is the inverse of time. This would turn the energy equation into , where is the fundamental time unit and is an integer. A good contender for this would be the Planck time, ~ seconds. To get to optical frequencies (~ Hz), you would need time quanta. At these frequencies, a 1 unit change in equates to a change in the 15th decimal place in frequency.



    That's picking femtohertz out of hundreds of terahertz. That is not presently possible, and so far, not of practical importance. So, it's fine for practical purposes to treat frequency as a continuum.
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