# Is there a difference between the speed of light and that of a photon?

As in the title I am curious whether there is a difference between the speed of photon and the speed of light, and if there is what is the cause of such a difference?

• Well... it depends how you define 'light'? Generally, this definition is in terms of photons, so there can be no difference by definition. – Danu Oct 12 '14 at 8:04
• Light as generated by the sun – Gobabis Oct 12 '14 at 8:49
• That's not a precise enough definition. – Danu Oct 12 '14 at 8:50
• Light can be thought about either as an electromagnetic wave or as a photon. This idea is called the wave-particle duality, and applies to light, where the name photon is given to a "particle" of light. I would suggest having a read on wave-particle duality, electromagnetic waves and quantisation of light if you are still confused. I hope this is helpful. – Robbie Oct 12 '14 at 9:21

It seems relevant to mention that there in principle could be a difference between the universal speed limit constant $c$ (which is usually casually referred to as the speed of light in vacuum), and the actual speed of light in vacuum if the photon has a rest mass, at least from an experimental point of view. Of course, no non-zero rest mass of the photon has been detected so far. See also this Phys.SE post and links therein.