Answer:
Approximately [tex]1.3\times 10^{8}\; \rm m\cdot s^{-1}[/tex].
Explanation:
Look up the speed of light in vacuum: [tex]c \approx 3.00\times 10^{8}\; \rm m\cdot s^{-1}[/tex]. Denote the speed as [tex]c[/tex].
If the speed of light in a medium is [tex]v[/tex], the refractive index of that medium would be:
[tex]\displaystyle n = \frac{c}{v}[/tex].
Note, that the refractive index of a medium is inversely proportional to the speed of light in this medium. A medium with a larger refractive index would thus correspond to a slower speed of light.
Rearrange this equation to find the speed of light [tex]v[/tex] in this medium:
[tex]\begin{aligned}v &= \frac{c}{n} \\ &= \frac{3.00\times 10^{8}\; \rm m\cdot s^{-1}}{2.4} \\ &\approx 1.25 \times 10^{8}\; \rm m\cdot s^{-1}\end{aligned}[/tex].
