Define refractive index of a medium.

Explanation

### Step-by-Step Solution:

1. **Definition of Refractive Index**: The refractive index of a medium is a measure that describes how light propagates through that medium.

2. **Symbol for Refractive Index**: It is commonly denoted by the symbol \( \mu \).

3. **Formula for Refractive Index**: The refractive index is defined mathematically as:
\[
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