# When light is reflected, is there ever an instant where it is at rest or is reflection instantaneous?

If a photon travels at $$+c$$ velocity and reflected to $$-c$$ it follows that there must be $$0$$ in between these velocities, just as if you threw a ball at a wall. But that presents a problem, as light can never be at rest since it is massless? Is the light absorbed and re-emitted at the surface? How exactly does this process work?

When the wave reaches the boundary, its shape rapidly changes. It doesn't get "absorbed and reemitted", there's no moment when the string stops moving. But for a while, it doesn't have a well-defined velocity in any direction. Something analogous happens for photons. When we say that photons always have speed $$c$$, we mean that plane waves of the electromagnetic field in free space travel at that speed (and hence so do wavepackets built around a given wavevector $$\mathbf{k}$$). This idea breaks down when you hit a boundary.
Thus, the string wave is always a superposition of a part that travels with uniform velocity to the right, and to the left. So if you wanted to think about a photon this way, then during the reflection, it's in a quantum superposition of going in and out, with all elements of the superposition having speed $$c$$. But perhaps at this point it's better to step away from the idea of speed here: it's only making things more confusing, and an idea that does that isn't worth keeping!