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What does a single photon look like

  1. Aug 31, 2006 #1


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    I've always wondered what is considered to be 1 photon. We always see things like "When two of these particle collide, they produce this and that particle, and ONE photon." The hell does that look like?!

    Is it like a monochromatic plane wave but just a line.. like a frozen segment of a sine wave propagating at c in some direction..?
  2. jcsd
  3. Aug 31, 2006 #2
    i think photon is like a "pulse".... a sine wave "bump"....

    well, what do i know...
  4. Aug 31, 2006 #3

    Claude Bile

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    Come to think of it, what does any particle look like?

    Is an image of a particles wavefunction the best we can hope for? Though some would argue that the wavefunction IS the particle.

    Quantum Physics was never my strong suit.....

  5. Aug 31, 2006 #4


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  6. Sep 2, 2006 #5
    I think this is like asking what does a tree look like when nothing is looking at it. I am guessing what something looks like will depend on how we decide to 'look' at it.
  7. Sep 2, 2006 #6


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    Sure, someone might look at a tree from the top and say a tree is a gree circle. Someone looking at it from the ground would say it is a green ball elevated by a brown trunk. Both would then have a definition of a tree which would enable them to recognize a tree when they see one.

    The same should be true about photons. If physicists say "When an electron and an anti-electron collide, the result of the collision is a photon.", it must be that they have some way to recognize a photon, otherwise, how could they be making this affirmation.

    What are then, these characteristics of a photon? What does it look like as seen from a bubble chamber (besides being invisible and caracterised by certain reactions that involves it, what else can we say about it)? Etc.

    But what interests me the most are its electromagnetic properties.
  8. Sep 6, 2006 #7
    Light and photon are particular substances exerting chemical reaction on our eyes;What does it look like seems to be more likely a biological than a physical question;We should now make another question,maybe what a photon appears?
  9. Sep 7, 2006 #8


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    A single photon looks like a poker hand before it is dealt. It's a probability function - a concept, not an entity with predictable, deterministic properties.
  10. Sep 7, 2006 #9
    photon? thats simple, its just a thing which is two things, yet one thing. what can be simpler?
  11. Sep 7, 2006 #10


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    Electron-positron annihilation produces two gamma rays.

    Photons interact differently with matter (usually atomic electrons) than charged particles do. That's how we tell the difference between gamma rays and beta or alpha particles.
  12. Sep 8, 2006 #11
    To realise the apearance of photon,i think the pic which quasar has for its name is a very good imagination.its energy in some shape but when seen from different angles give different pictures of a same things(e.g.a sphere looks like a circle when seen from a side).OR
    its nothing but STILL ALOT.it is there ,but u cant see it though there is not vaccum or cavity there.
    good idea:its like air ,which we cant see ,but we know its there(though experimentally proved)
  13. Oct 31, 2008 #12


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    Doesn't your correction to the particulars simply adjust the question, which now might be phrased as, "What does a gamma ray look like."
  14. Nov 1, 2008 #13
    It does seem that a particle “looks” like whatever the detector is designed to do when the particle enters. For example, when radiation such as a photon, enters an ionizing chamber it ionizes one or more gas molecules. The ions are attracted to the oppositely charged electrodes; where their presence causes a momentary drop in voltage, which is recorded by an external electrical circuit. The amount of voltage drop helps to identify the radiation because it depends on the degree of ionization, which in turn depends on the charge, mass, and speed of the photon. And all of these properties are pre-determined by the mathematical physics. In other words, the physicists calculate what a photon’s properties are, then set a trap for one based on those properties. If a particle arrives that exhibits said properties, it is deemed to be a photon.
  15. Nov 1, 2008 #14


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    This seems to be something that is being over-analyzed. The retina of the eye records the impact of a single photon. That flash of light is what a photon looks like.
  16. Nov 1, 2008 #15


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    Last edited: Nov 1, 2008
  17. Nov 1, 2008 #16
    In keeping with the spirit of “over analyzing” this subject: :tongue:

    There are two types of receptors on the retina at the back of the human eye. Colour vision is possible because of the cones while vision in low intensity light is possible because of the rods. In an experiment performed in 1942, it was established that on average, an individual rod was sensitive to a single photon! However this does not mean that we can consciously detect a single photon. In fact, if our eyes were that sensitive, our brain would be swamped with noisy stimuli. Indeed, we have evolved to the stage where at least between five to nine photons must reach the rods within a time period of one hundred milliseconds before filters will activate a signal to the brain and for us to be conscious of seeing something [1].

    It is also important to note that only about 10% of the light that reaches the eye actually is received by the rods: 3% is reflected from the cornea, 47% is absorbed by various non-sensing parts and an incredible 40% actually falls between the rods and goes undetected. Therefore one deduces that between 500 to 900 photons must arrive at the eye every second for our brain to register a conscious signal.[2]
  18. Nov 1, 2008 #17


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    Very cool post, Schroder.
    I knew that the retina is sensitive to a single impact, but I hadn't heard anything about the filtration system that causes the brain to ignore it. Thanks for the info.
  19. Nov 1, 2008 #18
    This is the only image I could find.

    Attached Files:

  20. Nov 1, 2008 #19


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    Rieke and Baylor, Single-photon detection by rod cells of the retina, Rev. Mod. Phys, 1998: "...Sakitt asked her observers to rate the strength of each and every applied flash on a scale of 0–6 rather than indicating only whether the flash was seen. .......... Sakitt’s work suggests further that the rods produce signals that allow neurons in the brain to discriminate between 1, 2, and 3 absorbed photons, and her experiments give an estimate of the rate of photonlike noise events that limit the reliability of this discrimination."

    So I think you were right the first time round :smile: - except for the over analysis part. Apparently it was realised in the early 1900s that we might see single photons, but it took another 60 years :wink: of experiments to make that our current view.
  21. Nov 1, 2008 #20
    But, is that our current view?

    "The human eye is very sensitive but can we see a single photon? The answer is that the sensors in the retina can respond to a single photon. However, neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 ms. If we could consciously see single photons we would experience too much visual "noise" in very low light, so this filter is a necessary adaptation, not a weakness.
    Some people have said that single photons can be seen and quote the fact that faint flashes from radioactive materials (for example) can be seen. This is an incorrect argument. Such flashes produce a large number of photons. It is also not possible to determine sensitivity from the ability of amateur astronomers to see faint stars with the naked eye. They are limited by background light before the true limits are reached. To test visual sensitivity a more careful experiment must be performed."


    [1] The Sensitivity of the Human Eye:

    Julie Schnapf, "How Photoreceptors Respond to Light", Scientific American, April 1987
    S. Hecht, S. Schlaer and M.H. Pirenne, "Energy, Quanta and vision." Journal of the Optical Society of America, 38, 196-208 (1942)
    D.A. Baylor, T.D. Lamb, K.W. Yau, "Response of retinal rods to single photons." Journal of Physiology, Lond. 288, 613-634 (1979)
  22. Nov 1, 2008 #21


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    The review by Rieke and Baylor I cited depends on Sakitt's 1972 work to suggest that single photons influence perception. They obviously have reservations drawing a stronger conclusion. I don't think the review's available free, but a detailed discussion is available on another review on Greg Field's site, on which Rieke is also an author:

    Field, Sampath and Rieke
    Retinal Processing Near Absolute Threshold: From Behavior to Mechanism
    http://www.snl.salk.edu/~gfield/Site/Welcome_files/Field2005.pdf [Broken]
    Last edited by a moderator: May 3, 2017
  23. Nov 2, 2008 #22
    I'm skeptical. A grain of silver nitrate in photographic film can require several photons to trigger the grain to blow up. This depends on grain size. Film can by bathed in weak light to 'sensitize' it without setting off too many grains prematurely.

    But the claim does sound reasonable, if the dyes within the eye are small enough in size; it would also be reasonable if they are single molecules.

    As I recall, sensitized film can remain sensitized for years. How do these researchers know that these spots of dye have not been sensitized?
    Last edited: Nov 2, 2008
  24. Nov 4, 2008 #23


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    The receptor cell in the human eye contains huge amounts of light sensitive molecules. presumably one photon exciting one molecule would not be enough to trigger an electric discharge to be sent via the neural network. There is a clear need for more photons. Taking into account the incredible number of photons that pass through our iris we would be simply blinded if it was not so.( even with reflections and diffraction )

    If i would imagine a photon it would look like this: the X spacial axis would be the direction; the Y (or Z does not matter) would present an electrostatic tension at it's maximum for the photons given energy. This electric tension then triggers a magnetic tension on the Z axis (Y if inverse) that rises to the point that the electrostatic becomes 0 in a sine function. the magnetic tension then triggers the electrical etc... electric to magnetic and back would be one wavelength. It's speed is then determined by the permeabilty and permeativity of the medium.
  25. Nov 4, 2008 #24
    You cannot say anything meaningful about a photon in between the time it is emitted and the time it is absorbed. If you are aksing what does it look like once we have "detected it" by some means, it always looks like a particle.
  26. Nov 4, 2008 #25
    Personally I think this question has no true answer just like my old bio teacher said once on the subject of sound waves "What defines sound? Sure the waves kind of do,but is it not just waves until registered by human or animal ears?" ok so the retina is very sensitive and we can not consciously see photons but does that mean that they actually look like somthing until we DO see them? I mean the electron microscope able to see atomic structure rendered in a 2-d image does that truely mean that the picture we all love to visualize of a bunch of spheres huddled together surrounded by a ever going cloud of smaller spheres really a atom? or is it wat we perceive to be an atom because as humans usually do we wish to classify it. To end my rant i say this "Why do we classify a photon? Why not keep it simple as this; We need it to see."
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