What actually are photons, means are they packets of energy or some sort of matter.
There is a nice thing called "Google" on the internet. Use the search term "Photons".
Photons are the quantized modes of electromagnetic field radiation.
Mathematically, this means that they are a way to describe classical light in a quantum mechanical way; a similar analysis can be made for sound, resulting in the phonon.
The photon represents a quantum state; there are definite rules that apply, which depend upon the situation. Planck's relation is very general, and relates the frequency of the classical radiation to the energy of the photon: if you know the frequency, the energy is also known: energy = Planck's constant x frequency, or E=h*f.
From an observational perspective, photons in large groups act just about how you would expect light to act: they show interference and diffraction, can deliver energy, and have momentum. They always travel at the speed of light.
But as you have fewer and fewer their unique quantum properties become more apparent: when detected, all of the energy appears at a single point, so they behave as though they are a particle. This is the origin of the photoelectric effect: light is absorbed by a metal basically one "lump" of energy at a time, based on the energy from Planck's relation. Thus if the electrons require a certain amount of energy to escape from the metal (the "work function"), there will be no electron emission for light consisting of photons with energy below that cut-off point - which results also in a frequency cut-off. This is why the photoelectric effect is independent of the intensity of the light.
There is a lot more that can be said; I refer you to Richard Feynman's book & lecture, "QED: strange theory of light and matter". The four-part lecture series can be found on the web.
Wikipedia is usually pretty good as a starting point: http://en.wikipedia.org/wiki/Photon
Photons are particles that "carry" the electromagnetic field energy. When they give energy to another particle (such as an electron) they do so by transfering the energy in discrete steps of E=h*f.
Though they are particles their (rest) mass is zero. I know this may sound absurd but thats one of the features of the modern (20th century) physics, that there can be particles with zero mass yet not zero spin or electric charge or some other properties.
Thanks for all the replies friends. According to them, Photons are the quantized modes of electromagnetic field radiation, or merely electromagnetic field energy, so can it be able to bend space-time according to General Theory of Relativity.
Don't hang your hopes on ever finding a 'cuddly' description of the Photon that makes it 'like' anything else you know. People have wasted hours of their time, trying to picture what a photon is like and, even worse, they have spent hours of their time trying to 'explain' to others what they think. Photons are quantum particles and that means that they cannot be explained in non-quantum terms. You just end up with loads of paradoxical statements and a heated argument.
This is just an abstract entity - model specific; something like a number in the algebra theory, or a point, line and a vector in the geometry.
In the reality we can identify a photon as an event - the detection of impulse, energy, a state change... but this is rather oversimplification.
The photons have nothing to the GR, which is just a geometric representation of a gravity force-field; this is quite different model and rather incompatible with the QM.
But how is the photons related with the Maxwell Equation??? Equation???
Not in any direct way. The wave equation that you get from Maxwell is a classical one. It tells you nothing about the 'nature' of a photon (a quantum entity). It can give you information about the probability of a photon interaction in a particular location, though.
Your dilemma is the same as everyone else's.
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