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