Does light have an acceleration?

[Jay-qu]

everything i have learnt leads to me believing that light has an instantaneous acceleration to C, but i thought i would ask anyway...

 

consider the situation of a light beam striking an object and rebounding back (i dont know if it is the same photon but i will assume it is...) then acceleration equals change in velocity over time - that is if newtons laws of motion can apply to a massless peice of energy... can someone help me out here please?

[Kirk Gregory Czuhai]

one can NOT apply Newton's laws ONLY in the case you are considering, one must use special relativistic equations and even some quantum mechanics!

 

The total relativistic energy of a body is:

 

E=mc^2=sqrt[p^2c^2+m0c^4]

 

and from the Planck quatum relationship, E=hf, and c=(f)(w)

 

one will get that IF the photon is Absorbed with no reflection, then the momentum change is

 

p = h/w where, h is planck's constant and w is the wavelength of the light

 

and if the photon is reflected, then the momentum change is TWICE THIS VALUE,

 

and since,

 

from NEWTONS SECOND POSTULATE,

 

its NOT, JUST F=MA, BUT

 

F= d p/dt

 

one can get the force exerted by the photon on the object,

 

but it really makes no sense to talk about the acceleration of the photon because it exists ONLY

at the speed c, and the reflected (in the case of reflection) photon is not the same photon as it has different quantum numbers (its going in the opposite direction)

 

THE MORE I WRITE THE MORE CONFUSING I GET!

 

SORRY! hope this helps a little...

[Jay-qu]

yeah answers a few questions, but like you said the more you say the more confusing it gets... and im only doin yr12 physics at the moment so im still a learner...

 

so if you turn on a torch the light generated by electrical energy will leave the wire at C instantaneously - this makes sense, but it is just another one of those wierd phenonemon that i think has a bit more to it...

[UncleAl]

Photons propagate at lightspeed - identical for all inertial observers. Photons in a medium with positive refractive index still propagate at lightspeed. They are being absorbed and re-emitted. Know the difference between phase and group velocities.

 

Relativistic physics does not recognize mass as such. It considers 4-momentum. Newton was wrong in the particulars. Get over it.