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Question about properties of light?

  1. Jun 5, 2009 #1
    So I know light is wave-particle. What is the wave part (interference,dispersion,diffraction)? What is the particle part(refraction, reflection, E=hf)?
    Also what is the difference between the intensity of light and the energy of light?
    One more, in my book it says doubling frequency would not double energy but affect intensity. how is this possible?
    Thanks for any help
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  3. Jun 5, 2009 #2


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    The particle part is that you can detect individual photons.
    Quantum mechanics solved the confusion by showing that everything is a wave and particle - it's just that the wavelength is small for larger objects, which is why we only see the wave effects on very small things like photons and electrons.

    The energy of light is normally taken as the energy of an individual photon, so blue light has more energy than red light (shorter wavelength=high frequency=more energy).
    Intensity is the total power = energy of photon * number of photons / second.
    So doubling the frequency would double the energy of each photon and if you had the same number of photons it would also double the intensity.
    Intensity and energy are not necessarily linked. You could have a very dim blue LED with high energy photons but a small number of them (a fraction of a Watt) and an infrared cutting laser with low energy photons but a lot of them (many 1000W)
  4. Jun 5, 2009 #3
    So this question is the reason I am so confused.
    A certain metal plate is completely illuminated by a monochromatic light source. Which would increase the number of electrons ejected from the surface of the metal.
    1. increase the intensity of light source
    2. increase the frequency of light source
    the answers is only #1.
    They explain bc increasing the frequency increases the energy which just gives the electrons more kinetic energy as they leave the metal not more leave.
    What am I missing in understanding this? How does an electron eject rather than absorb energy? And if it absorbs energy does it become vibrational leading to heat?
  5. Jun 5, 2009 #4


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    This is a special feature of metal surfaces.
    An electron can only be ejected from a surface if the energy of a single photon is enough to free a single electron. You cannot use lots of low energy photons to do the same thing - it's one of the original proofs of Quantum Mechanics that energy can only be exchanged in fixed sized packets.

    Once a photon has more than enough energy to break an electron free - the extra energy is given to the kinetic energy of the electron.

    This isn't the same question as asking about the intensity/energy of a light beam.
    Last edited: Jun 5, 2009
  6. Jun 5, 2009 #5
    Or very cold things!

    BEC's are my favorite part of quantum physics that i hope to one day fully understand
  7. Jun 5, 2009 #6
    So for ejecting an electron, the energy of the photon determines if the electron can be ejected. And the intensity is the amount of photons (if monochromatic) of the same energy hitting the atom/surface.
    With a light beam, increasing frequency or decreasing wavelength increases energy of the single photon and intensity. Correct?
  8. Jun 6, 2009 #7
    Sorry, just a quick question. When a body cools down by radiating heat radiation does the body cool down by reducing its vibration by a fixed amount and emitting fixed sized photons? Or can any size photons be generated at anytime equivalent to the decrease in vibration?
  9. Jun 6, 2009 #8
    And, if that photon has more energy than it needs for that purpose, the extra energy would go toward giving the electron some kinetic energy as it moves away. If the photon has exactly enough energy to free the electron, but no more than that, the electron will be freed from the metal but not have any velocity away from it.
  10. Jun 6, 2009 #9
    The energy of any one photon is proportional to frequency, and inversely proportion to wavelength. The amplitude of the electromagnetic wave is how many photons have been emitted. Intensity is energy per unit area per unit time, so it would increase if you increase the energy of any one photon, or if you increase the number of photons, or both. The way you put "and intensity" at the end of the sentence, the grammar confuses me, so I'm not certain that the answer to "correct?" is "yes", and I have just edited this post four times.
    Last edited: Jun 6, 2009
  11. Jun 7, 2009 #10
    Maybe I can extend my question.
    Do phonons turn into photons when a body reduces vibration and radiates heat?
    Are phonons only of quantified sizes?
    Are photons that were phonons (if they were) of fixed quantified sizes?
    Side question: Do phonons create gravity?
  12. Jun 7, 2009 #11
    I'm pretty sure this is incorrect. We may be able to create single photons, but there is nothing new in the detection process that lets us really verify this.

    The experimental work on single photons is a more or less recent development (last 20 years or so). In the early years of quantum mechanics, the only light sources were basically thermal in nature. With such sources, it's really hard to tell when you have exactly one photon. Lasers aren't really much better in that respect. Eventually, people developed special ways of generating light which theoretically was supposed to consist of precisely one photon at a time. This is what people talk about when they say we now can deal with individual photons.

    But the detection process is still the same as it always was. You can't tell at the point of detection if you are seeing general illumination from a thermal source or whatever.
  13. Jun 7, 2009 #12


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  14. Jun 7, 2009 #13


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    Sure you can, there are many types of detectors that can do this. This is more or less standars lab-equipment, at least for high-frequencies (and can be done for lower frequencies as well; but then you need specialized equipment).

    Also, the problem with determinging what type of source generated the photon has nothing to do with the electronics, in order to actually verify that they were generated by a true single photon source you need to do some careful measurements looking at the correlation function. Even thermal photons will give rise to discrete events on the detector.

    Generating single photons is actually more difficult; but it is also possible (although I don't know if there are any commercial products that can do it yet).
  15. Jun 7, 2009 #14
    How do you focus a single photon towards the detector?
  16. Jun 7, 2009 #15


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    Lenses and mirrors.
    You can find "full" QM descriptions of e.g. mirrors in many textbooks on quantum optics.

    Most of the time the physics works out be identical to what you get from a classical description. There are however some notable differences; for e.g. a 50/50 mirror (a "half-silvered mirror") you need to take all FOUR ports into account; the vacuum at the "unused" port actually has an effect; in a classical description one could just ignore it since there is no such thing as a "classical" vacuum state.
  17. Jun 7, 2009 #16
    There are detectors which respond to very weak illumination.

    There may possibly be detectors which are able to reduce the number state of an incident field by a count of exactly one.

    Er...but I do not believe there is a detector which can tell the difference between a single photon state and a classical electromagnetic wave. That was the original question, remember: how does one know if light is a particle or a wave?
  18. Jun 7, 2009 #17


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    The detector on its own can't tell the difference; but by measuring the correlation functions using for example a Hanbury-Brown&Twiss interferometer one can tell the difference between thermal light, coherent (="classical") light and light coming from a single photon source (only true single photon sources will have g(2)=0 for t=0).

    You can find several review papers that explain this in more detail, e.g.
    "Single-photon sources"
    M Oxborrow, AG Sinclair - Contemporary Physics, 2005
  19. Jun 7, 2009 #18
    So using some combination of lenses and mirrors is how they get a source which produces just a single photon to send each single photon to the detector?

    So using this method the single photon can be going in any direction initially from the source and will end up at the detector without fail?
  20. Jun 7, 2009 #19


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    There is no such thing as 100% perfect equipment; but as far as I know the "passive" components that are in use are quite good. There are of course other ways of doing it, e.g. use an optical fibre (which is how long-distance quantum cryptography is usually tested) or, if one is working at lower frequencies, a waveguide.

    As far as I know there is actually little difference between how one handles single photons and many photons from a purely practical point of view.
    That said, none of my work is done at optical frequencies so my knowledge about how the experiments are actually realized is very limited (although I share equipment and lab space with people who work with single optical photons)
  21. Jun 7, 2009 #20
    Cool. That must be fulfilling to work with such equipment. Thanks for what you could give me.
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