If light is made of photon particles and the photon doesn't have any mass but it is a form of energy (according to my thinking) then why do we call photons particles?
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3$\begingroup$ Why do you think particles must have mass? $\endgroup$– innisfreeCommented Mar 28, 2019 at 7:36
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$\begingroup$ As a student I think that particle is being referred to a entity which has some mass $\endgroup$– Amresh Prasad SinhaCommented Mar 28, 2019 at 7:39
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1$\begingroup$ But you know that photons are massless and referred to as particles? Why does it make sense for you to only call massive things particles? $\endgroup$– innisfreeCommented Mar 28, 2019 at 7:41
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$\begingroup$ That's what I am asking here....that if photon doesn't have mass then why we refer it as a particle $\endgroup$– Amresh Prasad SinhaCommented Mar 28, 2019 at 7:41
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$\begingroup$ There are many similar questions and answers here. As it happens, I addressed this question in another context here on PSE just yesterday. Since it's fresh in my mind, I point you to it. $\endgroup$– garypCommented Mar 28, 2019 at 18:49
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3 Answers
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One should probably not say that a photon is a particle (or that it is something at all), but rather that it can successfully be described using particle properties in some circumstances.
A particle can have several properties (e.g. mass), but for a photon, this does not apply. Particle properties of a photon might include indivisibility and a position in space - but only after a measurement.
One property that all particles share is that they are (in principle) countable, this is done in single photon experiments.
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$\begingroup$ I would say that a very important property of "particles" is countability. You can—at least in principle—say how many there are. And in quantum electrodynamics on a fixed metric you get that for excitation of the EM field. Of course, there is a problem when general relativity rears it's head, but that's just one of many difficulties in that arise in GR. $\endgroup$ Commented Mar 29, 2019 at 2:49
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In practical terms, we call photons "particles" because careful experiments reveal that they are 1) emitted singly from sources like excited atoms, 2) interact singly with things like photoelectric detectors, and 3) possess repeatably well-defined energies. All these things mean that photons can be considered particles in some circumstances (usually where they can be studied one at a time), despite the fact that they also possess wavelike properties in other circumstances (usually where there are many of them present).
answered Mar 28, 2019 at 18:35
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$\begingroup$ Re (1): it is exceptionally hard to emit single photons from a source. $\endgroup$– JasperCommented Mar 28, 2019 at 19:20
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1$\begingroup$ @jasper, by "source" I meant an excited atom. Will edit. $\endgroup$ Commented Mar 28, 2019 at 19:30
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Light when interacts with matter behaves as it made of particles just like when alpha particles collides with an electron and absorbs the energy of the electron, there is a similar kind of phenomenon when light interacts with matter as in photoelectric effect when an electron of the cathode absorbs energy from light and comes out of the metal. This phenomenon is same as any kind of particle particle interaction, so it is assumed that light consists of particles when it interacts with matter and this particle is known as photon. There are no rules to define a particle on the basis of mass
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$\begingroup$ Can you elaborate it more plz $\endgroup$ Commented Mar 28, 2019 at 8:38