What will be the refractive index of a medium in which light travels with the speed of $1.4\mathrm{ }\times \mathrm{ }{10}^8 \mathrm{m}/\mathrm{s}$? Given that the speed of light in a vacuum is$\mathrm{ }3\mathrm{ }\times \mathrm{ }{10}^8 \mathrm{m}/\mathrm{s}$.

Given, 
Speed of light in vacuum, $c= 3 \times {10}^8\mathit{ m}/s$
Speed of light in medium, $v= 1.4 \times {10}^8\mathit{ m}/s$
It is known that the refractive index is given by the ratio of the speed of the light in a vacuum to the speed of the light in the medium. Mathematically,
$\mu =\frac{c}{v}$
where $\mu$, $c$ and $v$ are the refractive index, speed of light in vacuum, and speed of light in medium respectively.
On substituting the values, we get the refractive index as
$\Rightarrow \mu =\frac{3 \times {10}^8}{1.4\times {10}^8}$
$\Rightarrow \mu =2.14$
Hence, the refractive index is $2.14$.