NCERT Class 10 Science — Chapter 1: Light - Reflection and Refraction

mcqmedium

Which one of the following equations correctly defines the absolute refractive index (n) of a medium?

Official previous-year question

Verified 1 Aug 2026.

Options

An = v/c
Bn = c − v
Cn = c/v
Dn = c·v

Did you get this right?

Solution

The correct option is n = c/v.

Explanation

The absolute refractive index of a medium is a measure of its optical density, which determines how much the speed of light is reduced when propagating through it compared to a vacuum. It quantifies the bending of light as it enters the medium from a vacuum.

Analysis:

The absolute refractive index ($n$) is mathematically defined as the ratio of the speed of light in a vacuum ($c$) to the speed of light in the medium ($v$).
The relationship is expressed by the equation:
$n = \frac{c}{v}$
Since the speed of light in a vacuum ($c \approx 3 \times 10^8$ m/s) is the maximum possible speed in the universe, $c$ is always greater than or equal to $v$. Consequently, the absolute refractive index is always greater than or equal to 1 ($n \geq 1$).
n = c/v correctly represents this definition. Options n = v/c, n = c·v, and n = c − v are incorrect formulations.

Key Takeaway:

The absolute refractive index is a dimensionless quantity given by $n = c/v$, indicating how much slower light travels in a medium compared to a vacuum.

Your note

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The correct option is n = c/v.

Explanation

Analysis:

The absolute refractive index ($n$) is mathematically defined as the ratio of the speed of light in a vacuum ($c$) to the speed of light in the medium ($v$).
The relationship is expressed by the equation:
$n = \frac{c}{v}$
Since the speed of light in a vacuum ($c \approx 3 \times 10^8$ m/s) is the maximum possible speed in the universe, $c$ is always greater than or equal to $v$. Consequently, the absolute refractive index is always greater than or equal to 1 ($n \geq 1$).
n = c/v correctly represents this definition. Options n = v/c, n = c·v, and n = c − v are incorrect formulations.

Key Takeaway:

The absolute refractive index is a dimensionless quantity given by $n = c/v$, indicating how much slower light travels in a medium compared to a vacuum.