Refractive index


Definition of Refractive Index of a Medium


The refractive index of a medium is defined as the ratio of speed of light in vacuum to that of speed of light in medium.

Refractive Index = Speed of light in vacuum / Speed of light in medium
300 000 Km / sec / 1.33
Refractive Index = Speed of light in vacuum / Speed of light in medium
Example: The Speed of light in vacuum is 300 000 Km/s and the speed of light in air = 200 000Km/s
Find the refractive index of the glass.
Refractive Index = Speed of light in vacuum / Speed of light in medium
300 000 Km/sec / 200 000 Km/sec = 1.5

More About Refractive Index


  • The medium with large refractive index will have large bending effect, when the light passes through them light refracts at larger angel to the normal.
  • Example: Refractive index of Diamond is 2.42 hence the diamond has greatest bending effect of light. It glitters.
    The light rays will slow down when passing through the diamond.
  • The medium with least refractive index will have less bending effect.
  • Example: Refractive index of water is 1.33 hence the water has least bending effect of light.
  • It is not possible to have a medium with refractive index less than 1, because light travels much faster in only vacuum.
  • Refractive index will not have any units as it is a constant.

Worked Example


Calculate speed of light in water.
Step 1: Refractive index of water = 1.33
Step 2: Speed of light in vacuum = 300 000 Km/s
Step 3: Refractive Index = Speed of light in vacuum / Speed of light in medium
Step 4: 300 000 Km/sec / Speed of light in water
Step 5: Speed of light in water = 300 000 Km/sec / 1.33 = 225563.90 Km/s

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