Refraction of light is change in direction of light while travelling from one medium to another. For example: pencil if placed half immersed in water appears to be bent. Not only pencil but anything kept half immersed in water appears to be bent. Moreover, extent of change in direction of light changes from medium to medium. For example, if very same pencil is half immersed in kerosene, bent observed will be different from bent observed in water.
Law of Refraction
Laws of Refraction
Refraction of light satisfies law of refraction, which states that
“The incident ray, refracted ray and normal to the boundary at point of incidence, all lie in same plane”
“The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for given color and given pair of media”
Sin i / Sin r = constant
This law is also known as snell’s law
Refractive index
From law of refraction, constant of Snell’s law is known as refractive index. Refractive index shows the extent to which the light changes its direction. Consider two different medium through which a ray of light travels, then refractive index of medium 2 with respect to medium is defined as “Ratio of speed of light in medium 1 to speed of light in medium 2” it is denoted by n21. Or
n21 = (speed of light in medium 1 / speed of light in medium 2) = v1 / v2
Therefore, refractive index of medium with respect to medium is defined as “Ratio of speed of light in medium 2 to speed of light in medium 2.” It is denoted by n12. Or
n12 = (Speed of light in medium 2/ Speed of light in medium 1) = v2 / v1
Absolute Refractive Index nm
Absolute refractive index is refractive index of medium with respect to air . i.e.
Nm = Speed of light in air / Speed of light in medium = c/v
Here c is speed of light i.e. 3 × 108 m/s. Medium can be water, glass, ice, diamond, etc.
For example, absolute refractive index of water is 1.33. This shows that ratios of speed of light in air to speed of light in water is 1.33.
Refraction of light- Relation between refractive index and bending of ray of light
Bending of Ray of Light
- If refractive index of first medium is rarer and that of second medium is denser then, speed of light decreases and the ray of light bends towards normal.
- If refractive index of first medium is denser and that of second medium is rarer, in such case speed of light increases and ray of light bends away from normal.
Keywords: Refraction of light, law of refraction, Snell’s law, Refractive index, Absolute refractive index
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