The speed of light in diamond is 
Explanation:
The index of refraction of a material is equal to the ratio between the speed of light in vacuum and the speed of light in the material:

where
n is the index of refraction
c is the speed of light in vacuum
v is the speed of light in the material
In this problem, we have:
n = 2.42 is the refractive index of diamond
is the speed of light in vacuum
Solving for v, we find the speed of light in diamond:

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