Refractive index is defined as
[tex]\mu = \frac{speed of light in air}{speed of light in medium}[/tex]
[tex]\mu = \frac{c}{v}[/tex]
given that
v = 0.6 c
so we will have
[tex]\mu = \frac{c}{0.6c}[/tex]
[tex]\mu = 1.67[/tex]
The refractive index is 1.66
The speed of light in a transparent medium is 0.6 times that of its speed in vacuum .
Refractive index of medium = speed of light in vacuum / speed of light in medium
So
RI = 1/0.6 = 5/3 or 1.66
