I think its Mercury because it's the closest to the sun.
So n=c/v, n= index, c=speed of light and v= speed of light in diamond. 2.42=c/v so v=c/2.42, c≈<span>3x108 m/sec</span><span> so v=</span><span>1.24x108 m/sec</span>.
<span>Hope this helps.</span>
