Question

the speed of light in a piece of plastic is 2.00x10^8 meters per second what is the absolute index of refraction of this plastic

Answer 1

The speed of light in a material with refractive index n is modified according to the following law:
$v= \frac{c}{n}$
where c is the speed of light in vacuum ($c=3\cdot 10^8 m/s$), while v is the speed of light in the medium ($v=2 \cdot 10^8 m/s$). Re-arranging, we can find the refractive index of this plastic:
$n= \frac{c}{v}= \frac{3 \cdot 10^8 m/s}{2 \cdot 10^8 m/s}=1.5$