I know about how the modern physics is built upon the two famous experiments: 1. DSE: Double Slit Experiment 2. MME: Michelson Morley Experiment

But these experiments are really give confusions to me to think about a photon, if I consider that as only a particle. So the following questions arise.

  1. How are we measuring the wavelength of a photon? I came to know that even for single photon we are computing energy based on its wavelength. So it is confusing to get the idea of wavelength for a single photon. Can we measure the wavelength of single photon? If yes, how?

  2. How are we measuring the frequency of a photon? As the energy of photon is purely depends on wavelength (\lamda), why do we need the term frequency in the first case. Why did we define the term frequency in terms of its wavelength and its linear velocity (c) for a single photon, if we know only about its wavelength and linear speed?

  3. How are we measuring the linear speed of a photon? I read the history of computing the light speed and I read about Maxwell's equations too. I am aware of my this question too. Just I want to know whether we are using only the wave nature to fix the light speed by using the interference and diffraction gratings. Or is there any other modern/ classical way to measure the speed of light apart from using wave nature (incl Maxwell's Electromagnetic relationship with light)?

  4. Finally and simply E= h f . How are we validating this great equation? From the previous questions this question can be easily understood.

  5. How does the linear speed of a photon contributing to its energy? I know the difference between the linear and angular speed an object. If I consider the photon as a particle, it can have angular velocity too. I am wondering how these classical mechanics failed there. Even if it is true, for the massless photon what is the meaning of kinetic energy?


closed as unclear what you're asking by Jon Custer, sammy gerbil, Norbert Schuch, CR Drost, Gert Jan 11 '17 at 0:19

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  • $\begingroup$ Various ways for the measurements. What is your real question? $\endgroup$ – Jon Custer Jan 10 '17 at 20:30
  • $\begingroup$ What is the "linear speed of a photon"? $\endgroup$ – ACuriousMind Jan 10 '17 at 21:07
  • $\begingroup$ Some of your questions may be answered here physics.stackexchange.com/questions/235203/… $\endgroup$ – HolgerFiedler Jan 10 '17 at 21:09
  • $\begingroup$ I have edited my questions clearly. Please let me know if you need any more details @JonCuster. $\endgroup$ – Edison Phoenix Jan 11 '17 at 18:30
  • $\begingroup$ Please read the fifth question and you can get why am I so keen about the linear velocity rather angular velocity. I also aware of photon is very tiny and considered as massless. But in my imagination let me allow to consider it can have angular velocity too. Please let me know if you need more details of my questions and answer my questions @ACuriousMind. $\endgroup$ – Edison Phoenix Jan 11 '17 at 18:35

Electromagnetic calorimeters measure the energy of a single (usually high energy) photon.


I don't really know a direct method to measure the frequency, (although it's possible, that such a method exists), but we can measure c, and measure the wavelength with diffraction or interferometer too, this way the frequency can be calculated easily. (This would require more than one similar photon )

For measuring of the speed of the light in vacuum, there are many methods, some involving the direct measurement of the flight time:


and it can be proven theoretically, for example from the Maxwell Equations, that all electromagnetic waves propagate with the same velocity. (at least in vacuum)

I don't really understand your last question. As you said, E=h*f. So, the energy of the photon is independent from it's speed. But, since photons have zero rest mass, you could say that all of their energy originates from their motion.

  • $\begingroup$ Frequency is equivalent to wavelength and can be measured by standard interference measurements (even double slit). $\endgroup$ – Alexander Jan 10 '17 at 22:57
  • $\begingroup$ Yes, of course, they are connected very strongly, but by interference the wavelength is, what matters, and if you want completely experiment based answer, you have to measure c too, to make use of f*lambda = c $\endgroup$ – b.Lorenz Jan 11 '17 at 11:48
  • $\begingroup$ @Alexander As the energy of photon is purely depends on its wavelength (\lamda), why do we need the term frequency in the first case. Why did we define the term frequency in terms of its wavelength and its linear velocity (c) for a single photon, if we know only about its wavelength and linear speed? $\endgroup$ – Edison Phoenix Jan 11 '17 at 18:37
  • $\begingroup$ @b.Lorenz My previous comment to Alexander says my intention more clearly. $\endgroup$ – Edison Phoenix Jan 11 '17 at 18:41
  • 1
    $\begingroup$ @EDISONPHOENIX , its easier to measure wavelength than frequency. Another comment - a single frequency photon is completely delocalized. In order to construct a localized photon you must use some frequency band, and thus the single photon is in superposition of many energies at some band. Here is a good discussion - physics.stackexchange.com/questions/273032/… $\endgroup$ – Alexander Jan 11 '17 at 18:42

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