Question

The absolute refractive index for a mineral is defined as the refraction relative to that in air. Then its value for different minerals ranges between:

• A. 1 and 1.2
• B. 1.3 and 2
• C. 2 and 3
• D. 3 and 4.5

Hint:

Refractive index = how much light bends; most minerals: 1.3–2.0 range.

Answer 1

The Correct Option is B

To determine the correct range for the absolute refractive index of minerals, let's analyze the optical properties of minerals:

1. Definition of Absolute Refractive Index:
The absolute refractive index (n) of a mineral is defined as:

n = (speed of light in vacuum or air) / (speed of light in the mineral)

2. Typical Ranges for Minerals:
Most rock-forming minerals have refractive indices between:

• Lowest: 1.3 (e.g., cryolite, n=1.338)
• Highest: ~2.4 (e.g., rutile, n=2.4-2.9)

The vast majority fall in the 1.3 to 2.0 range.

3. Why Other Ranges Are Incorrect:
- 1-1.2: Too low (only gases approach n=1)
- 2-3: Too high (only rare minerals exceed 2.0)
- 3-4.5: Impossible for natural minerals

4. Common Examples:
- Quartz: 1.544-1.553
- Calcite: 1.486-1.658
- Olivine: 1.63-1.87

Final Answer:
The correct range is 1.3 and 2.