Question

speed of light remains constant in the universe then why it changes in mediums
like in diamond the speed of light is almost half(1.5×10^(8)m/sec)​

Answer 1

Bonjour!¡

Since different mediums have different refractive indexes and speed and refractive index is closely related, the speed of light is different in every medium.

Answer 2

Due to the phenomena of ....refraction of light....

When light ray passes from rarer(air) to denser medium(i.e, diamond)

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