I read the stackexchange question on the topic but the answers are too complex for me
In: Physics
because light is a continuous electromagnetic field.
Hardly ELI5, but imagine two close points in space, and be also imagine they had a slightly different frequency. While one point is oscillating, eventually the other point (as it is faster) will end up one cycle ahead, and later even more cycles ahead.
How would the continuous(!) field in-between look like? it could not! Therefore the frequency must be the same everywhere.
The frequency of a photon is a property of the photon, the amount of energy with which it was created.
Refraction preserves this property, or else the process of focusing light would change its color. To understand how, you need to look at how light passes through a “transparent” medium.
In a vacuum, it’s easy to be transparent. The photon moves along, unimpeded, because there aren’t any atoms to interact with. In water or glass, things are different. A photon interacts with the electrons around an atom. This interaction exchanges energy back and forth between the photon and the electron. It’s complicated, but in the end the photon (actually a new photon, but we’re ignoring photon serial numbers here) ends up with all the energy it started with. This allows the light to pass through without being absorbed†. Since it leaves with the energy it started with and the frequency depends on the amount of energy inside, the light remains the same color.
† Of course, some is absorbed because water and glass aren’t perfectly transparent like vacuum is. However an individual photon is either absorbed or not.
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