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Why does the a light wave crest, rather than continuing off in the direction it's heading prior to the moment it begins cresting?

What force/acceleration/thing 'turns' the wave towards the bottom (or top) so that it changes direction at the crest or bottom of the wave, reversing direction?

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    $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Sep 5 at 20:31
  • $\begingroup$ I'm not sure if this is what you're thinking, but light as a wave is not a particle travelling in a trajectory that looks like a wave. $\endgroup$
    – Jonas
    Sep 5 at 21:10
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    $\begingroup$ I think he is asking what causes the B and the E fields to keep on alternating back and forth in an EM wave. $\endgroup$
    – Tachyon
    Sep 5 at 21:11
  • $\begingroup$ Good question and good reasoning. Of course nothing is going back-and-forth as it moves at the speed of light. Light is made of billions of individual photons. The light wave is made of the billions individual coherent photons. You’re better off picturing each one of those coherent photons oscillating in phase as they propagate at the speed of light. $\endgroup$ Sep 5 at 21:58
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Why does the a light wave crest, rather than continuing off in the direction it's heading prior to the moment it begins cresting?

A light wave does not involve any particle moving with an oscillatory trajectory.

What's "waving" in a light wave is the magnitude and direction of the magnetic (B) and electric (E) fields.

One reason neither the B nor the E field can increase in magnitude indefinitely is that these fields store energy proportional to the square of their magnitude. So infinite energy would be required for the fields in a finite volume of space to increase indefinitely.

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