Question

Why refractive index had no unitt​

Answer 1

it is a unitless number because it is the ratio of speed divided by speed. in other words m/sec divided by m/sec. the units cancel leaving just a unitless number.

Answer 2

bro simply it is ratio

and ratio has no units

for example 5m/10m

it is also a ratio of distance

so m/m cancel out

hence ratio has no units

I hope it is helpful to you