The speed of light in a material is given by:
[tex]v= \frac{c}{n} [/tex]
where
[tex]c=3 \cdot 10^8 m/s[/tex] is the speed of light in vacuum
n is the refractive index of the material
The lens in this problem has a refractive index of n=1.50, therefore the speed of light in the lens is
[tex]v= \frac{c}{n}= \frac{3 \cdot 10^8 m/s}{1.50}=2 \cdot 10^8 m/s [/tex]
And the correct answer is C).
