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]
