# Relativistic mass of photon in motion [duplicate]

There are two expressions regarding relativistic mass of photon; namely relativistic approach:

$$m = \frac {m_0} {({1 - \frac {v^2 } {c^2 }) } ^\frac {1 } {2} }.$$

And, energy-mass equivalence formula:

$$E = m {c}^2.$$

There common uses in physics books:

First formula is used to prove that rest mass of photon is zero.

Second formula is used to find out what is called kinetic mass of photon in motion.

MY QUESTION: first formula suggests that photon must have infinite mass in motion, and second gives finite mass (kinetic), why is this disperacy?

None of the formulae you quote can be used to conclude that the mass of a photon is zero. The formula that should be used to do so is the generic special relativistic relation $E^2 - p^2c^2 = m^2c^4$ in conjunction with the formula relating the energy and momentum of a photon $E=pc$. Putting this latter formula into the former immediately yields $m=0$, i.e. that mass of a photon is zero.
• To be clear, the symbol $m$ in your answer denotes the invariant mass while the symbol $m$ in the OP's question does not. This might lead to some confusion. – Alfred Centauri Nov 27 '17 at 17:53