The highest refractive index among the following is :
A. glass
B. water
C. diamond
D. ruby

Hint: Refractive index $\eta$ of a substance is the ratio of the velocity of light in vacuum $c$ to its velocity in a specific medium $v$.$\eta=\dfrac{c}{v}$, this is a constant value. Also the higher the refractive index the slower the light travels in that medium, which increases change in the direction of the light within that material.

Complete step-by-step solution -
Refractive index $\eta$ of a substance is the ratio of the velocity of light in vacuum $c$ to its velocity in a specific medium $v$.$\eta=\dfrac{c}{v}$. It is a dimensionless constant.
The higher the refractive index the slower the light travels in that medium, which increases change in the direction of the light within that material. Also when light travels from lower to higher refractive index material, light bends towards the normal. Similarly, the lower the refractive index, the faster the light travels in that medium, which decreases the change in the direction of the light within the material, when light travels from higher to lower the refractive index, light bends away from the normal.

The refractive index of the above given substances are as follows:
For glass: 1.5
For water:1.33
For ruby:1.77
For diamond: 2.42
Hence, C.diamond, has the highest refractive index among the following.

Additional Information:
The two types of refractive index are :
1) Absolute refractive index, where the ratio is taken between the speed of light in vacuum to speed of light in medium,and
2) Relative refractive index , where the ratio is taken between the speed of light in two mediums other than vacuum.

Note: Refractive index $\eta$ of a substance is the ratio of the velocity of light in vacuum $c$ to its velocity in a specific medium $v$.$\eta=\dfrac{c}{v}$, there are two types of refractive index, absolute and relative. Here absolute is taken into account. The refractive index of a material is a constant and is useful to remember them. This is described by Snell's law of refraction, $\eta_{1}\sin\theta_{1}=\eta_{2}\sin\theta_{2}$