Q.What is a photon? State any two properties of a photon that show it behaving like a particle rather than a spread-out continuous wave.

Definition. A photon is a discrete packet of electromagnetic energy, of magnitude $E=h\nu$ for light of frequency $\nu$, that light is absorbed and emitted in, rather than continuously (Section 11.7-11.8).

Two particle-like properties. (1) It carries a definite momentum, $p=E/c=h\nu/c=h/\lambda$, exactly as a moving particle does, even though it has no rest mass and always moves at speed $c$. (2) In an interaction such as the photoelectric effect, its ENTIRE energy is transferred to a single electron in one indivisible event -- it is not absorbed gradually, piece by piece, the way a continuous wave's energy would be, and it conserves energy and momentum with the electron exactly as an ordinary particle collision would. …

Define the photon by its energy formula, then list two properties (momentum, indivisible energy transfer) that …

• Saying a photon 'has mass' -- a photon has zero rest mass; its momentum comes from $p=E/c$, not from $p=mv$. …