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How do we measure a photons frequency?

  1. May 8, 2004 #1
    G'day all, my first post :)

    I've recently been studying quantum physics (well the basics anyway) and I have come across parts I don't understand fully.

    We define a photon as the discrete quantity (particle) of electromagnetic radiation. What I am trying to understand is exactly how to picture a photon. If it is a particle what is this frequency a measure of?

    Through my study I've seen the need to introduce the photon to explain black body radiation as classical theory did not work. How exactly is this the case?

  2. jcsd
  3. May 9, 2004 #2


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    To your first question, the most convenient image would probably be this:
    when thinking of a photon as a particle, frequency is a measure of how many of these particles pass a given point ni a given amount of time.
  4. May 9, 2004 #3
    I was under the impression that all photons, irrespective of energy, travel at speed C. Would that not result in an equal amount of photons passing a given point in a given time (for the same intensity of the light), nomatter the frequency of the photons.

    Also energy can be defined per photon, how then does your image differentiate between 1 low energy photon and 1 high energy photon passing a certain point?
    Last edited: May 9, 2004
  5. May 9, 2004 #4


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    This isn't correct. The "frequency" of light corresponds to the rate of oscillation of the E (and B) fields. The "measure of how many of these particles pass a given point" is the light's intensity. These were illustrated in the photoelectric effect.

  6. May 9, 2004 #5
    You mention the E and B fields, but how do these fields exist as far as the photon is concerned?
  7. May 9, 2004 #6
    If I may extend a guess. The frequency is a measure of self interaction. If a photon is a geometric with direction - It will have a front and a back, and the interaction of these two will be expressed in frequency. The front pulls while the back is pulled {{Then flip the coin}} The back pushes while the front is pushed. This implies that one half of a photon is moving faster than C, while the other half is traveling slower than C {{Another flip of the coin}} and the operation is reversed. Average speed of a photon is C.

    I look at a photon as being the shape of a salad bowl that increases in size. The exterior of the bowl is the front of the photon, while the interior of the bowl is the back. The bottom of the bowl is the foci of the photon. From there - Imagine waves eminating from that location like ripples produced from a stone thrown in a pond. As the ripples extend outward, so does the bowl increase in size. This could be termed the extension of the photon. A photon would take this shape (bowl) because no part of it can travel faster or slower than C (average speed) from it's point of emision.
  8. May 9, 2004 #7
    Lurch gave you an incorrect answer, while Ultrapi1 is just venturing his own theory without having studied the subject. Only zapper gave you a correct answer. I think it is most productive to first read what very intelligent physicists have discovered during the last century and before. The first very successful description of light was by Maxwell, who considered ligth as a propagating oscillation in the electromagnetic field. Then we had a new and better description in the 20th century provided by quantum mechanics. Actually, Maxwell's description and the quantum-mechanical description are quite compatible. From a forum like this you can get a few opinions, but if you really want to understand this topic you will have to continue reading from books. If you don't like the very mathematical treatments, you can read some popularizations first.
    Maxwell's equations describe light as electromagnetic radiation which consists of oscillations (waves) in the electric and magnetic fields which are always perpendicular to each other. This theory explains things like refraction, interference and polarization but does not tell you anything about photons.
    Quantum mechanics describes light as emitted and absorbed in discrete bundles of energy called photons. Here the waves are interpreded more like probability waves (this is not exactly correct but an oversimplification). In a beam with a large number of photons, the probability translates into density of photons. This very rough description may give you the impression that these waves imply an ondulation in the density of photons along the beam. This is not so. The density usually remains constant (unless there is absorption). When the beam goes through two closely spaced slits, on the other side of the slits you do get regions of high and low photon density (light intensity). This phenomenom is called interference and is produced by the interplay between the two beams that went trough the slits.
    The frequency of the oscillations in a beam of light is proportional to the energy in each photon, as demonstrated by the photoelectric effect, and in the case of light is related to the color of the light.
    The intensity of the beam is proportional to the number of photons.
    The polarization of light (that is explained by Maxwell) is related to the quantum-mechanical concept of spin. You can see the photon as a little top spinning around an axis that coincides with the direction of propagation. But while in clasical mechanics an object can spin only in one direcetion at a time, in quantum mechanics you have the paradoxical and counter-intuitive fact that an object can spin lets say clockwise and counterclockwise at the same time. Is like having two "realities" existing at the same time. It takes a while to get used to this new idea and to accept it. A photon spinning in one direction corresponds to a rotating electric field, and to what is called circular polarization. A photon that spins in both directions at the same time gives you, under the right circumstances, plane polarization, which means the electric field is oriented always in the same direction. You can find nice pictures in the books that show you the rotation of the vectors for circular and plane polarization. What they don't always make is the connection between the classical picture and the the quantum-mechanical concept of spin, which I understand was one of your concerns.
    If I recall correctly, the text Optics by Hetch gives a good explanation of the connection between the classical and the quantum-mechanical pictures of light. I don't remember if the name was exactly "Optics" but you can do a search in Amazon.com with Hetch and Optics and you'll find it.
    Good luck in your studies,

    Oh! how do you measure a photon's frequency?
    Prepare a beam of identical photons and , if they correspond to light from the ultraviolet to the near infrared (including the visible) pass it through a prism as in the typical experiment by Newton. Light of different frequency (wavelength) will be defflected by different angles. You can also use a grating instead of a prism.
    Last edited: May 9, 2004
  9. May 10, 2004 #8


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    One bit of irony that I noticed regarding the frequency: For a given intensity, the frequency of the E&M radiation is inversely proportional to the number of photons that pass in a unit of time. In other words, higher frequency photons require less per unit time for a given intensity.
  10. May 11, 2004 #9
    Many thanks alexepascual, your description helped me quite a bit :)
  11. May 11, 2004 #10
    That's true. To make clear to Jaguar why this would be so: Ligth intensity is measured in terms of energy flow (flux). For a particular beam intensity, if each photon packs more energy, you need less photons to transport the same total energy.
    I am glad I was able to help.

    Last edited: May 11, 2004
  12. May 24, 2004 #11
    Does anyone have a good real example/ photograph of the two-slit experiment 'interference pattern' as all the books I've seen show approximations?
  13. May 24, 2004 #12


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    I'm not sure what you mean exactly by "approximations" of these interference pattern. The 2-slit experiment with monochromatic light (typically a He-Ne laser) is a very common experiment in an intro physics undergraduate laboratory exercise. So this isn't just a "good real example", it is also done and demonstrated frequently.

  14. May 27, 2004 #13
  15. May 27, 2004 #14


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    Those are what they observed.

    And what wavy lines?

  16. May 27, 2004 #15
    In a previous post to this thread I made a mistake when recommending an optics book. The name of the author is "Hecht" and not "Hetch".

    "Wavy lines" may be used for two purposes. They may be used to represent the advancing wave of light and they may be used as a graphical representation of the light intensity on the screen (intensity of the interference fringes)
    The book "Optics" by Hecht has a picture on page 341 and a very good mathematical analysis of the phenomenom.
    You may also find interesting the recent post by KingNerd04, who tried to do the experiment himself.
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