The speed of light in a material with refractive index n is modified according to the following law:
![v= (c)/(n)](https://img.qammunity.org/2019/formulas/physics/high-school/xun5uphw3w9m94muwgtv2mcr3cz994ip8p.png)
where c is the speed of light in vacuum (
![c=3\cdot 10^8 m/s](https://img.qammunity.org/2019/formulas/physics/college/ljf7to97i4azjztupparr278vwfzuapv8g.png)
), while v is the speed of light in the medium (
![v=2 \cdot 10^8 m/s](https://img.qammunity.org/2019/formulas/physics/high-school/b0fb9u80trech3hh1p70nxo2adpp801qq4.png)
). Re-arranging, we can find the refractive index of this plastic:
![n= (c)/(v)= (3 \cdot 10^8 m/s)/(2 \cdot 10^8 m/s)=1.5](https://img.qammunity.org/2019/formulas/physics/high-school/57pqzc1eybml5igiqb9nf6kjvezysgk37d.png)