If light has no mass, how can it have energy according to e=mc2?

[lightarrow]

How then would you explain to them that one photon has rest mass = 0 and two of them (not traveling exactly in the same direction) have total rest mass =/= 0?

Taken as a system, the average velocity (vector) of the two photons you describe is now less than c.

Regards,

Bill

It's not clear to me how you define the average velocity (vector) of the two photons in this case.

 

[Antenna Guy]

It's not clear to me how you define the average velocity (vector) of the two photons in this case.

Project the individual velocity vectors into an average inertial frame.

Regards,

Bill

 

[lightarrow]

Project the individual velocity vectors into an average inertial frame.

Regards,

Bill

Sorry, but do you mean a normal projection along a coordinate axis? That is, if the velocity vector of every photon makes an angle theta with a coordinate axis then you take c*cos(theta)?

 

[Antenna Guy]

Sorry, but do you mean a normal projection along a coordinate axis? That is, if the velocity vector of every photon makes an angle theta with a coordinate axis then you take c*cos(theta)?

Sort of - v*cos(theta) is the same thing as a vector dot product where one vector (i.e the new direction) is of unit length. The "v" in this case is the magnitude (or length) of a velocity vector - not the vector itself.

Additionally, adding two mutually orthogonal unit vectors perpendicular to the new direction vector would define what I called an "inertial frame". Using vector dot products in the same way, these vectors can be used to account for velocity components not in the new direction of propogation - vis. v*sin(theta)*cos(phi) and v*sin(theta)*sin(phi).

If the "average direction" is accurate, it should be consistent with the net kinetic energy of the system.

Regards,

Bill

 

[rohanv2]

Who the hell just calls the Light has no mass!

There are Photons (particles) in light hence being conserved that light has mass. If it has momentum then it is sure that it has mass hence is mass = 0 then

P= m X v
= 0 X V
= 0 kgm/s ?? wth?

Pst.
Einstein proved E=mc² by the help of momentum of light.

After the light that hits the object the electrons are ejected out which and then the object becomes visible but the amount in which it ejects out the electrons are in very less amount but though [ as told by our teachers ], then it definitely needs energy. While if the mass = 0 then
E= Nm
= m X a X s
= 0 X a X s
= 0 Joule??
How can light light participate even in photoelectric effect without any energy to eject out electrons?

I guess this is enough for proving that light has mass... :D

 

[Dale]

Hi rohanv2, welcome to PF

Calm down, this thread has been dead almost two years now. It really is just a question of definition, a pulse of light has "relativistic mass" but no "rest mass". So pick your favorite flavor and relax.

 

[rohanv2]

Hi rohanv2, welcome to PF

Calm down, this thread has been dead almost two years now. It really is just a question of definition, a pulse of light has "relativistic mass" but no "rest mass". So pick your favorite flavor and relax.

Indeed thanks and sorry! didn't realize that it's dead already. Anyway, I realized that light has no mass, it is just a form of a wave (just a sort of flow of energy). How silly of me. :D