What is refractive index ?

What is refractive index ? Correct Answer it is defined as the ratio of speed of light in vacuum to the speed of light in the medium .

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পানির প্রতিসরাঙ্ক (Refractive index ) 1.33 - হলে পানিতে আলোর বেগ কত?
The refractive index of the ocean water:
Which of the following has the highest value of refractive index?