Can the absolute value of the refractive index be less than unity?

Answer: No, the absolute value of the refractive index cannot be less than unity.
The refractive index (denoted by nn) of a medium is a measure of how much the speed of light is reduced inside the medium compared to its speed in a vacuum. It is defined as:
n=cvn = \frac{c}{v}
Where:
• nn is the refractive index,
• cc is the speed of light in a vacuum (approximately 3×108 m/s3 \times 10^8 \, \text{m/s}),
• vv is the speed of light in the medium.
Key points about the refractive index:
1. Refractive index in vacuum: For a vacuum, the refractive index nn is equal to 1, because the speed of light in vacuum is cc.
2. Refractive index in other media: For any other medium, the refractive index is greater than or equal to 1. This is because light always travels slower in any material medium compared to a vacuum, meaning vv is always less than cc. Therefore, the refractive index in any material is greater than or equal to 1:
n=cv≥1n = \frac{c}{v} \geq 1
o In transparent media (like glass, water, etc.), the refractive index is greater than 1.
o In optically rarer media (like air), the refractive index is close to 1 but slightly greater than 1.
3. Refractive index less than 1: A refractive index less than 1 is not physically possible for normal materials. This would imply that light travels faster in the medium than in a vacuum, which contradicts the fundamental principle that light cannot exceed the speed of light in vacuum in any medium.
Special Cases:
• Negative Refractive Index: In some special materials known as metamaterials, negative refractive indices can occur, but these are highly engineered materials and do not occur in natural substances. Even then, the absolute value of the refractive index is still greater than or equal to 1, though the sign may be negative.
Conclusion:
For normal materials and natural substances, the absolute value of the refractive index is always greater than or equal to 1. It cannot be less than unity.
Thus, the correct answer is: No, the absolute value of the refractive index cannot be less than unity.