At what point does light come into existence from when it leaves its source?

  1. A torch (flashlight US) is positioned on a train going at 70kms/hour. Relative to someone standing beside the train track the torch will have a velocity of 70kms/hour since it is on the train. When you switch on the torch, a light beam emanates. However, the light was not actually inside the torch, it emanates from the torch. Light is made of photons. Do the photons exist in the torch or only outside the torch? If so, can it be said that the light beam is also on the train or did it only start to exist outside of the torch and therefore was never "on the train"?

  2. jcsd
  3. Astronuc

    Staff: Mentor

    Light (photons) are produced by excited electrons falling to lower levels within an atom. The atoms in the filament in the torch are producing light, to the light starts within the torch and propagates from the torch. Also, there is a reflector surrounding the filament, so some light moves laterally or backward in the torch before moving out of the torch.
  4. Light is a vibration - very much like waves on water.

    If you stick your hand in a pond and make ripples, it's exactly the same.
    So, do the waves exist in your hand? If not, where did they come from? and where do they go when they reach the shore?
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