The absolute refractive index of any transparent medium (defined relative to vacuum) is always:

Introduction / Context:
The refractive index of a medium describes how much the speed of light is reduced when it passes from vacuum into that medium. The absolute refractive index is defined as the ratio of the speed of light in vacuum to the speed of light in the medium. This question tests your understanding of this definition and asks you to identify a general property of the absolute refractive index for any real transparent medium compared with vacuum.

Given Data / Assumptions:

• Absolute refractive index n is defined as n = c / v.
• c is the speed of light in vacuum.
• v is the speed of light in the medium.
• We consider ordinary transparent media such as air, water, and glass.

Concept / Approach:
Light always travels fastest in vacuum. When it enters any material medium, interactions with atoms and molecules cause it to travel more slowly on average. Therefore, v, the speed of light in the medium, is always less than c, the speed of light in vacuum, for any transparent material. Because the refractive index is defined as n = c / v, and c > v, the ratio c / v must be greater than 1. If n were exactly 1, it would mean that v equals c and the medium behaves like vacuum. A value less than 1 would require light to travel faster in the medium than in vacuum, which contradicts established physics.

Step-by-Step Solution:
Step 1: Write the definition of absolute refractive index: n = c / v.Step 2: Recall that c, the speed of light in vacuum, is the maximum possible speed of light.Step 3: In any real transparent medium, the speed of light v is reduced due to interactions with the medium, so v < c.Step 4: Since c / v is a ratio with numerator larger than the denominator, n = c / v must be greater than 1.Step 5: Conclude that the absolute refractive index of any transparent medium is always more than one.

Verification / Alternative check:
Check typical refractive index values from tables: air has n approximately 1.0003, water about 1.33, and common glass about 1.5. All these values are greater than 1, consistent with the definition. A medium with n exactly 1 would behave like vacuum for light, and none with n less than 1 exist in normal circumstances, as that would imply superluminal speeds. Advanced physics may describe some unusual effective indices in special materials, but in standard optics for real transparent media, n is always greater than 1. This confirms the general statement required by the question.

Why Other Options Are Wrong:
Zero, option B, would imply c / v = 0, which is impossible because c and v are both positive speeds. Exactly one, option C, would mean the medium does not slow light at all and is equivalent to vacuum, which is not true for ordinary transparent materials. Less than one, option D, would suggest light travels faster in the medium than in vacuum, contradicting basic principles of relativity and optics. Only option A, more than one, is consistent with the definition and observed values of absolute refractive index.

Common Pitfalls:
Some students misinterpret absolute refractive index as a relative measure between two media and forget that the definition here is specifically with respect to vacuum. Others may have seen approximate values close to one, such as for air, and mistakenly think that values less than or equal to one are common. To avoid confusion, remember that in this context absolute means relative to vacuum, and vacuum provides the reference speed c. Because no medium allows light to exceed this speed, the ratio c / v is always greater than one.

Final Answer:
More than one