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Q.
The minimum value of relative refracting index is:
Ray Optics and Optical Instruments
Solution:
The refractive index of medium b w.r.t. medium a is given by
$\,{}^an_b=\frac{speed \,of \,light\, in\, a}{speed \,of \,light \,in \,b}$
Since, the speed of light in vacuum is the maximum, therefore, in principle, the refractive index of vacuum w.r.t any other medium is less than 1. However, in general, we define refractive index of different media w.r.t. vacuum and these values are greater than 1. The refractive index w.r.t. vacuum is often called absolute refractive index.