What is a photon exactly?

What actually does 'frequency of a photon' mean?

• This struck me while walking home today. http://i50.tinypic.com/2ni6scm.jpg so this is the general 'representation' of a photon moving forward at the speed of light. Time period t, f = 1/t. However, the photon doesn't move up-down-up-down etc, or else it'd have a SPEED greater than c (VELOCITY would equal c), so it moves in a straight line. This then confused me. If f = 1/time period, what is the time period of a photon that moves in a straight line, and what -is- the wavelength? Are they just values used to satisfy e=hf and v=fλ? Because from what I see, all a photon has is energy. Thanks for answering!

• Answer:

  Photon is not like a small metallic ball. You can't define photon's coordinate with uncertainty smaller then it's wavelength. So it's absolutely meaningless to speak about 'up-down' movement of a photon. A photon is characterized by momentum and energy. These quantities are related by well-known relativistic formula E^2=p^2*c^2 + m^2*c^4. Photon has m = 0, hence E^2 = p^2*c^2 or E = p*c. These are internal parameters of a photon. It's customary to speak about photon's frequency: E = h * nu, but nu has nothing to do with "up-down" motion of a photon. It's just another way to speak about photon's energy. And please do not mix classical field with a notion of a photon. Classical field is not a collection of a fixed number of photons (Fock states), but is a bizarre quantum superposition of such states (known as coherent states). So classical-quantum correspondence is not straightforward.