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Once the light is emitted as a pulse, and once that pulse has passed, what's behind it? Is light a continuous wave, or only a wavefront? What is left in the wake?

Qmechanic
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A "pulse" is a wave packet that starts and then ends. There is nothing in front of it, and nothing behind it. An example is what happens when you turn a flashlight on and then turned it off, leaving you in complete darkness.