Why does light slow down in glass but not stop completely?
Quick Answer
Light appears to slow down in glass because its electromagnetic wave interacts with the material, delaying its progress. It still travels extremely fast, but slower than in vacuum.
The Full Story
In a vacuum, light travels at about 299,792 kilometers per second. In glass, the electromagnetic field of the light interacts with the electrons in the material, so the wave’s effective speed is reduced. The light is not simply “bouncing between atoms”; rather, the combined wave response of the material changes its propagation. This is why light bends when it enters glass or water, an effect described by the refractive index.
Key Facts
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