Question

Light travels fastest in:
(A). Kerosene
(B). Turpentine
(C). Water
(D). All

Answer 1

Hint: The speed of light depends on the medium in which it is travelling. Light moves slowly in an optically denser medium. The refractive index is expressed as the ratio of the speed of light in a vacuum to the speed of light in that particular medium. So, if the speed of light in that particular medium is low, we would have a very high refractive index. So, lower the refractive index of the medium, higher will be the velocity of light.

Complete step by step answer:

The refractive index of the material is defined as the ratio of the velocity of light in a vacuum to the velocity of light in other mediums. The refractive index is always greater or equal to one. It is dimensionless in nature and is denoted by ‘n’.
$\text{n}=\dfrac{\text{c}}{\text{v}}$
Where,
n is the refractive of a particular medium.
c is the velocity of light in vacuum.
v is the velocity of light in a particular medium.
A medium having a higher refractive index is called an optically denser medium, a medium which is having a lower refractive index is called an optically rarer medium. Light travels fast in an optically rarer medium. If we consider the refractive of materials given in the options, we can see that the refractive index of turpentine is 1.525, the refractive index of water is 1.33 while the refractive index of kerosene is 1.35. So from these values, it is clear that the refractive index of water is lower than any refractive index. So light will travel faster in water.
Light shows the maximum velocity in a vacuum. So there is a vacuum in the options, you can be sure that it is the medium through which light travels faster. Air also has a refractive index very close to one (n=1.003) but not less than vacuum.
So the answer to the question is option (C)- Water.


Note: The speed of light in a vacuum is exactly 299792458 meters per second, which can be approximately written as $3\times {{10}^{8}}\text{ m/s}$. Light has the maximum velocity in vacuum. Light shows the maximum velocity in a vacuum. So there is a vacuum in the options, you can be sure that it is the medium through which light travels faster. Air also has a refractive index very close to one (n=1.003) but not less than vacuum.