Question

The ratio of the velocity of light in vacuum to the velocity in a given media is called ………….
(A). Normal Index
(B). Reflective Index
(C). Refractive Index
(D). None of the above

Answer 1

Hint: The ratio of the velocity of light in vacuum to the velocity in a given media tells us about how much the velocity of light decreased while travelling through a medium.

Complete step by step answer:
First of all, let us get a very clear idea about what the velocity of light means and how it changes depending on different media of its propagation. The word ‘‘velocity’ determines the distance travelled by an object or matter at a particular instant of time. Velocity of light is however special for the fact that the velocity of light in vacuum is a universal constant. This means that, anywhere in the universe, the velocity of light in vacuum will remain constant, that is, it will not change, if considered at different places of the universe at different instants of time.

Important: The value of the velocity of light in vacuum is $299792458$ metre per second (${m}/{s}\;$).

Now, we should be aware of the fact that the velocity of light decreases while travelling through a medium. So, we need a quantity which can determine the amount by which the velocity decreases while propagating through a medium. Thomas Young, in the year $1807$, determined a quantity which could give us information about the change in the velocity of light while travelling through a medium and he named the term as, ‘index of refraction’ or ‘refractive index’.

Definition- Refractive Index of a material is a dimensionless quantity which gives us a measure of how fast light travels in a particular medium.
Mathematically, we can write,
$n=\dfrac{c}{v}$
where, $n$ represents the refractive index of a particular medium, $c$ represents the velocity of light in vacuum, $v$ represents the velocity of light of the medium for which we need to determine the refractive index.

Therefore, from the above discussion we arrive at a conclusion that the ratio of the velocity of light in vacuum to the velocity of light in a medium is known as the ‘refractive index’ of the medium.

Therefore, the answer of the given question is, option C. Refractive Index

Additional Information:
Refractive index of a medium also determines how much the path of light is bent, or refracted, when entering a material. Some of the important values of refractive indices which is required to do several problems are,
Refractive index of water is $1.33$.
Refractive index of glass is $1.52$.
Refractive index of ice is $1.51$

Note: The velocity of light in vacuum is a universal constant, but it may change if light propagates in any medium other than vacuum. As light moves from one material or medium to another, the velocity of light increases or decreases according to the density of the medium.