Chapter 26 - Geometrical Optics - Problems and Conceptual Exercises - Page 941: 50

$1.25$

We can find the index of refraction of the substance as follows: $n=\frac{c}{v}$ $\implies n=\frac{c}{\frac{d}{t}}$ We plug in the known values to obtain: $n=\frac{(3.0\times 10^8)(4.0\times 10^{-9})}{0.960}$ $n=1.25$