Is optical density of a medium same as its mass density ? Name a transparent medium which has smaller mass density, but is optically denser.

Explanation

To solve the question, we need to analyze the concepts of optical density and mass density, and then provide an example of a transparent medium that fits the criteria given.

### Step-by-Step Solution:

1. **Understanding Mass Density**:
- Mass density (ρ) is defined as the mass of a substance per unit volume. It is usually expressed in units like kg/m³.
- Formula:
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