Final answer:
Light travels the slowest in glass due to refraction.
Step-by-step explanation:
In a vacuum, light always travels at the same speed, which is approximately 299,792 kilometers per second (km/s). However, when light passes through different mediums, such as air, water, or glass, it can be slowed down.
The substance in which light travels the slowest is glass. When light enters glass, it interacts with the atoms and molecules in the glass, which causes it to slow down.
For example, when light travels from air into glass, it slows down and changes direction, a phenomenon called refraction. This is why objects can appear distorted when viewed through a glass lens.
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