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Physics/regarding photons

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Question
Hi,

What is the experimental proof that photons have zero mass?. How could a massless particle can have both particle/wave duality? Please explain. When light is reflected by an object and we see through our eye, how the photons capture the image of the object? What change happens in the photons after the reflection of the light?

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Answer
Aside from the ridiculous precision of relativity, which depends on massless photons, there have been many experimental attempts to determine if photons have mass.  Wikipedia details them OK for starters: http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass
However, people are combing through experimental particle physics databases, looking for evidence of some rare kind of photon with mass.  Thus far, no one has turned up anything.  They're well-motivated, since a photon with mass would mean serious new physics and major fame for the discoverers.

Anyhow, when light is reflected, image formation is determined by the fact that photons travel in straight lines.  If you just check here: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/mirray.html you can see some examples for mirrors.  Lens examples also exist for refracted light.  During reflection, only tiny amounts of energy/momentum occur during the absorption and re-emission of photons (the "reflected" photon conserves these quantites, but is technically absorbed and re-emitted during the process as a new photon when considered on a deeper quantum level).

The wave/particle "duality" question I get a lot.  The misunderstanding originates with the word "particle," for which we have an intuitive misunderstanding.  At the quantum level of physics, "particle" just means the thing which is a whole unit.  In the case of light, it means the photon.  The photoelectric effect proved that light waves are not absorbed or emitted partially, only as whole and indivisible units.  That means that they are particles.  That's all the word particle means, that they must be absorbed or emitted as wholes and that you can't just slice off part of the wave.  Einstein was the person who figured this out, which makes his genius in a time when this was not understood or accepted (because of the instinctive quandry that the word "particle" seems to invoke) all that much greater.

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Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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