Refractive Index:
Refraction is the bending of light as it passes from one medium to another. The amount of bending depends on the nature of the two media and the angle at which the light strikes the interface between them. The refractive index of a substance is the ratio of the speed of light in a vacuum to the speed of light in that substance.

Formula:
Refractive index (n) = Speed of light in vacuum (c) / Speed of light in substance (v)

Calculation:
Given, the speed of light in the substance = 2×10^8 m/s

The speed of light in vacuum (c) = 3×10^8 m/s (constant)

Refractive index (n) = c/v
n = 3×10^8 / 2×10^8
n = 1.5

Therefore, the refractive index of the given substance is 1.5.

Explanation:
The refractive index of a substance indicates how much the substance slows down the speed of light. In this case, the speed of light in the substance is 2×10^8 m/s, which is slower than the speed of light in a vacuum (3×10^8 m/s). By dividing the speed of light in vacuum by the speed of light in the substance, we get the refractive index of the substance as 1.5. This means that the substance slows down the speed of light by a factor of 1.5.

The refractive index is an important property of a substance and is used in various applications such as designing optical lenses, determining the critical angle for total internal reflection, and measuring the concentration of a solute in a solution.

the ratio of the speed of light in vaccum to the speed of light in a medium is called refractive index of the medium.the formulae is n=c/vn= refractive indexc=speed of light in vaccumv= speed of light in medium.