Posted by **edward** on Sunday, August 11, 2013 at 1:34pm.

according to einstein's mass energy relation

M= m'/root[1-(v^2/c^2)]

Where M is mass of particle when it moves with velocity 'v'

And m' is its initial mass.

When particle moves with velocity of light i.e. When 'v=c' then

M= m'/0 =infinity i.e. Infinite mass

But a photon of light can have velocity of light.

So does this possess infinite mass?!!

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**Damon**, Sunday, August 11, 2013 at 4:22pm
no, it has zero rest mass m' so its mass is undefined at any speed.

As you know of course it does have finite energy and momentum.

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**Damon**, Sunday, August 11, 2013 at 4:24pm
see Wikipedia:

" .... In empty space, the photon moves at c (the speed of light) and its energy and momentum are related by E = pc, where p is the magnitude of the momentum vector p. This derives from the following relativistic relation, with m = 0:[15]

E^{2}=p^{2} c^{2} + m^{2} c^{4}.

The energy and momentum of a photon depend only on its frequency (ν) or inversely, its wavelength (λ):

E=\hbar\omega=h\nu=\frac{hc}{\lambda}

\boldsymbol{p}=\hbar\boldsymbol{k},

where k is the wave vector (where the wave number k = |k| = 2π/λ), ω = 2πν is the angular frequency, and ħ = h/2π is the reduced Planck constant.[16]

Since p points in the direction of the photon's propagation, the magnitude of the momentum is

p=\hbar k=\frac{h\nu}{c}=\frac{h}{\lambda}. ......"

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