Question

Define refractive index of a medium in terms of speed of light.

Hint:

Remember that refractive index is inherently a comparative ratio of speeds. Because it divides meters-per-second by meters-per-second, it is completely unitless and strictly dimensionless.

Answer 1

Concept:
• The refractive index serves as an optical measure of how much a material actively slows down light passing through it.
• Vacuum represents the ultimate speed limit where light travels the absolute fastest.

Step 1: Provide the strict definition
The absolute refractive index (often simply called the refractive index) of a specific optical medium is strictly defined as the mathematical ratio of the speed of light propagating in a pure vacuum to the speed of light propagating in that specific medium.

Step 2: Provide the mathematical formula
Mathematically, it is perfectly expressed by the equation:
\[ n = \frac{c}{v} \]
Where:
$n$ is the absolute refractive index of the designated medium.
$c$ is the universal speed of light in a vacuum (approximately $3 \times 10^8$ m/s).
$v$ is the actual phase speed of light measured traveling inside the medium.
Because light strictly travels slower in any material medium than in a vacuum ($v < c$), the absolute refractive index is universally a dimensionless quantity always strictly greater than $1$.