What is the evidence for light being particles instead of waves?

[Dali]

To touch on your original question:

1.How much evidence is there proving light to be particles rather than waves?

I think the best example when light really behaves like particles is the Compton effect, see http://en.wikipedia.org/wiki/Compton_scattering . This experiment can easily be done as an undergraduate lab on a tabletop setup today. To demonstrate the effect one uses an x-ray source that shines x-rays directly on one scintillating crystal detector. Every now and then, a photon hits an electron in the crystal, and the energy of that electron can be measured. At the same time (well, after the photon traveled to your next detector!) you can measure the deflected photon at a specific angle from its original line of propagation. The deflected photon has lost the precise amount of energy that was transferred to the electron it collided with. I.e. the deflected photon has a longer wavelength than it originally had.

This effect it really impossible to explain with light being any kind of classical wave. Firstly, a wave would never kick just one electron like that. And a wave loosing energy should decrease its amplitude, not change its wavelength. Secondly, one can calculate the wavelength-shift as a function of the deflection angle using a simple billiard-ball collision model. This show that light (or x-rays) - in this particular case - really is behaving very similar to a classical particle, and not at all like a wave.

(The two-slit experiment is the other extreme example where light - in that particular case - displays its wave-like properties!)

 

[Simon Bridge]

The speed of light a property of space, kind of allows for it to ba a medium?

No.

Sound, for example, although not QM, can travel through various substances. Wouldn't those be mediums? And those also limit it's s[p]eed. I know it's not like that but..

... but it is a compelling analogy? Analogies are all very well - but physics has to have a better connection to reality than that. You keep making analogies that lead you in unhelpful directions.

Sound waves are composed of the substance they move through ... if air, then they are the motion of air molecules, if a solid, then the atoms of the solid are shifted from their equilibrium positions.

Sound waves travel faster through a solid than through air because the component parts are more tightly bound to each other - so a displacement of parts in one place strongly affects the other parts nearby.

Light is usually thought of as a disturbance in an electromagnetic field ... the exact kind of disturbance depends on the model being used. None of the models propose light waves composed of

I think all your questions so far have been answered accurately, if not to your satisfaction, and you have a lot of reading to do. You have yet to show that you have started any of that reading. Get back to us when you have.

Cheers and happy learning :)

 

[harrylin]

Hi to all
This may be a stupid question.

1.How much evidence is there proving light to be particles rather than waves? Like the double slit experiment, using a single photon. Are there other experiments with a similar outcome?
And 2. How sure are we that we are actually firing a single photon in the said experiment? (it could, for example, be just the smallest amount of light that could still be considered light.. )

It's not a stupid question at all. There is a joke that light is particles during the week but waves during the weekend. Particles can be modeled with waves; however common waves are modeled as consisting of vibrating particles. Consequently you quickly face the philosophy of those who happen to be reply your question. It may be more constructive to ask how successful the existing models handle the cases that you bring up in your questions.

 

[CLourensS]

It's not a stupid question at all. There is a joke that light is particles during the week but waves during the weekend. Particles can be modeled with waves; however common waves are modeled as consisting of vibrating particles. Consequently you quickly face the philosophy of those who happen to be reply your question. It may be more constructive to ask h ow successful the existing models handle the cases that you bring up in your questions.

Ah. This is good news. I have this concept or idea but need to learn more. My knowledge is woefully lacking, especially the math. I have some leads to follow now and material to study. This forum is the bees knees. Hopefully I can do a bit more than ask irritating questions.