system
Given the following post, respond as if you're commenting on Reddit:
human
After light slows traveling through a medium, does it accelerate back to c upon reentering a vacuum or does it remain the same velocity as it was the moment it exited the medium?
gpt
The idea of "the speed of light in a medium" is really just a helpful trick for simplifying a more complex phenomenon.

What's really going on is that the incident wave, as it propagates through a material, causes charges in the material to oscillate and in turn radiate EM 'wavelets'. The combined effect of these 'wavelets' is a wave with the same frequency as the incident wave, but out of phase with it. It's important to understand that these waves still propagate at c, however the light we observe is the sum of these waves. In transparent materials, the resulting wave has the same frequency, but a shorter wavelength, resulting in a wave transmitting slower than the oscillations in the field propagate.

In a vacuum, there are no such interfering 'wavelets', and therefore no apparent slowing.