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The wavelength of a photon

  1. Mar 5, 2013 #1
    So if an electromagnetic wave can have a wavelength measuring 100 km (an arbitrary measurement of course), why is a photon a point particle. Is it a point, or a varying size maxing out at 100 km? Are the perpendicular magnetic fields spanning 100 km at the peak of the cycle?
  2. jcsd
  3. Mar 5, 2013 #2
    Alright i just had another thought. Lets say our eye has a 1 cm visual radius. Now if we were tuned to see radio waves, and a 1 cm electromagnetic wave hit our eye, would we just see one solid "color"? Because there is no way we would be able to see multiple photons of 1 cm wavelength.
  4. Mar 5, 2013 #3


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    It is a point particle. It has no known inner structure and each detection event will take place at some well defined position. There is some uncertainty in that position, but uncertainty in position is not the same as size.

    Generally speaking wavelength usually has nothing to do with size. Consider a car going at roughly 30 m/s. You can assign a de-Broglie wavelength to it which will be in the range of (roughly) 10^-38 m. If wavelength was any relevant in terms of size, a car would give a better point particle than a photon in the visible. This is obviously nonsense.
  5. Mar 5, 2013 #4


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    I don't think it's really meaningful to talk about the size of a photon, unless you pick some very specific definition of size. If you shine a photon on a photographic film, then you got a dot whose size depends on the size of a film grain. Or if you shine on a CCD camera, one pixel might record a count, so you might regard the size of a pixel as the size. If you imagine that the photon has a certain size, then you might think that there would be a minimum size for a pixel, below which the photon hits multiple pixels. But the theory says that there is no minimum size, so you could think of the photon as a point particle in that regard. There's no practical way to make a pixel on a screen smaller than an atom, though, so it doesn't really matter.
  6. Mar 5, 2013 #5


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    Taking "point particle" too literally is prone to some problems. If you think of both electrons and photons as point particles, you might think that the probability of a collision between the two is zero. But actually, the collision has a cross section on the order of 10^-24 cm^2. https://en.wikipedia.org/wiki/Thomson_scattering
    You can't think of size of fundamental particles in the classical sense.
  7. Mar 5, 2013 #6
    Ignoring that a monochromatic radio wave is just "one color" anyway... the number of photons has to do with the intensity of the light, not the wavelength/frequency (see: Photoelectric effect). A "one photon" electromagnetic wave would be far, far too dim for the human eye to sense, even if we were talking about a wavelength in the human range of vision. Increasing the intensity of light (you can think of intensity/power as being related to wave amplitude) increases the "number of photons," but for monochromatic radiation, all of the photons will have the same wavelength/frequency-dependent energy.

    Think of a photon as a discrete "chunk" of electromagnetic energy (more formally, it's called the quantum of electromagnetic radiation), with an energy that is related to the wavelength/frequency of the radiation. So saying that light needs to be a certain intensity to cause a sensor (the eye, a CCD pixel, etc.) to perceive it, is equivalent to saying photons need to be hitting it at a certain rate.
  8. Mar 5, 2013 #7


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    For your eye to perceive 1cm waves, your eye would be something like an antenna. If a photon is absorbed by the antenna, you can't really say the exact position of the photon. It interacts with the antenna in a global sense. Now if you had a grid of antennas, then you could localize the photon to one specific antenna, but you wouldn't be able to localize it more than that. It's very weird, because somehow being picked up by one antenna means it won't be picked up by the other antennas, since the photon is only allowed to be absorbed once. But how it chooses is mysterious.
  9. Mar 5, 2013 #8


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    A photon is not either one. You are using classical terms to describe something that is not classical. It is a quantum object. Sometimes it shows particle-like properties (if you are looking for that) and sometimes it shows wave-like properties (if you are looking for that). but to say that it IS a particle or a wave is a misuse of terminology.
  10. Mar 5, 2013 #9
    Giving the specific answer to your question, size of the object has nothing to do with the wavelength.
    Considering the De-Broglie equation
    wavelenght = h/p

    So, size is totally irrelevant. The whole photon does not account for the wavelength. The only factors that matters are the 'mass' and 'velocity' of the particle.
  11. Mar 5, 2013 #10
    Oh ok. The only reason I used the eye example is because i was told that the tiny holes on the front if a microwave (appliance) door had a specific width so it would absorb any microwaves that had a bigger wavelength (looks like maybe a mm or two), and would pass any smaller, like visible light. Is this right?
  12. Mar 5, 2013 #11
    The holes in the radiation shield of a microwave (or any effective radiation shield) need to be significantly smaller than the wavelength of the radiation. A typical kitchen microwave produces radiation on the order of ~5 inches.
  13. Mar 5, 2013 #12


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    I suspect that some microwave radiation actually does make it through the mesh in the window of your oven, but only a minuscule amount that's not enough to matter as far as your health is concerned. Clearly if the mesh spacing were really wide, say 1 meter, the microwaves would have little or no trouble passing through. As the spacing decreases, the amount that passes through decreases.
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