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Refraction Of Light:- When a ray of light travels obliquely from one transparent medium to another, its velocity changes, due to which it gets deviated from its original direction at the surface separating the two media. This phenomenon is called refraction of light.

Refractive Index:-

1. Absolute Refractive Index:- The ratio of velocity of light in vacuum to the velocity of light in given medium is called absolute refractive index of that medium. It is denoted by η (Greek letter eta)

So η=c/v

2. Relative Refractive Index:- The ratio of velocity of light in medium 1 to the velocity of light in medium 2 is known as relative refractive index of medium 2 with respect to medium 1. It is denoted by η₂₁.

So  η₂₁=v₁/v₂