Light

• When light traveling in a vacuum enters a new transparent medium, such as air, water, or glass, the speed is reduced in proportion to the refractive index of the new material. That is, Speed of light in any medium = speed of light in vacuum/refractive index.
• Light has its maximum speed in a vacuum as the refractive index in a vacuum is 1.
• As the refractive index increases the speed of light in the medium decreases.

Source: NCERT