Photons never slow down; only the waves they create do
In short: When light enters water or glass, it appears to decelerate, causing the familiar bend of a straw in a glass. Yet, according to physics, individual photons never actually lose speed. The mystery lies in the difference between how light travels as a single particle versus a collective wave.
To understand this paradox, we must distinguish between two types of velocity: phase velocity and group velocity. According to Maxwell’s equations, the speed of light is determined by the properties of spacetime itself. In a vacuum, light moves at its maximum constant speed because there are no obstacles. However, when light enters a medium like water or glass, it encounters atoms containing charged particles—specifically protons and electrons.
As an electromagnetic wave, light consists of oscillating electric and magnetic fields. When these fields hit the charged particles in a substance, they induce their own secondary electromagnetic waves. These induced waves are also made of photons, which travel at the maximum speed (c). However, because the medium is dense, these new waves interact with the original wave through constructive and destructive interference.
The result is a 'summed up' wave. While every individual photon—both from the original source and those induced by the atoms—travels at the constant speed of c, the collective package (the group velocity) moves more slowly. This is why light refracts or bends: the overall wave pattern slows down due to interference, even though the underlying particles are never accelerating or decelerating.
This distinction explains a fundamental observation in physics: there is no sudden 'speeding up' of photons once they exit a glass block and return to air. Because the individual photons were traveling at the maximum speed throughout the entire journey, their velocity remains unchanged by the medium they passed through.
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Sources
- Summary by Intellect · source: How Does Light Slow Down in a Medium, if Photons NEVER Do?
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