Light being emitted from a source in vacuum, can photons accelerate ?
No, photons move at c the moment they are created.
Here we are again with that perennial problem that arises because the word "particle" is used in describing a Photon. What a pity they didn't invent a special, alternative word for the photon. It's what happens when very clever people try to get very clever ideas across to us mere mortals. They can't have conceived the problems they were injecting into Science education by that choice of word.
Acceleration is the time derivate of velocity and not of speed.
The local speed of light in vacuum is constant but the direction can be changed.
I agree that this is very likely. However, it should be clarified that the answer doesn't refer to the question as it has been asked.
How? Even in general relativity, where light can seem to curve, it follows geodesics - no acceleration.
No, light in vacuum cannot accelerate.
I suspected but didn't want to say, thanks for the clarification
should remove what I said earlier
but light bend near massive object, so it is accelerated.
It terms of coordinate acceleration in non inertial coordinates, yes. The speed in non inertial coordinates is also not limited to c, but can take any arbitrary value.
In inertial coordinates though, which exist only locally in curved space time, light doesn't change speed nor direction.
That's why I limited my statement above to the local speed.
Light emitted from a source in a vacuum accelerates when it enters (or leaves) a material with refractive index > 1.
Light doesnt really accelerate otherwise it would go beyond c.What changes is the phase velocity vp=c/n so when n<1 , vp>c.
Acceleration is the rate of change of velocity, so negative values of acceleration (slowing down) are permitted.
Velocity is a vector and so is acceleration. Slowing down doesn't imply negative acceleration components, as their sign depends on the coordinate system.
Thank you. If an object is going 2 m/s, and then slows down to 1 m/s, what is delta v?
+1m/s if you work with rockets.
How is that question related to the speed of light in vacuum, the topic of this thread?
My assumption is that when something slows down, it accelerates (because its velocity is changing). Hence my previous assertion that light accelerates when it enters a material with refractive index >1.
The title of the thread:
Light speed is constant.
When light strikes a mirror in a vacuum, it changes direction.
And, when light is emitted from a refractive material source into a vacuum, it speeds up.
Velocity is the vector of speed, the absolute value of the vector is constant there is no acceleration. Speed is constant.
There are no mirrors or refractive materials in a vacuum.
Light doesnt stop when is hits the mirror.
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