What are para-axial rays and marginal rays ?

Explanation

### Step-by-Step Solution:

1. **Understanding the Principal Axis**:
- The principal axis is an imaginary line that passes through the center of a spherical mirror or lens and is perpendicular to its surface. It serves as a reference line for analyzing the behavior of light rays.

2. **Defining Paraxial Rays**:
- Paraxial rays are those light rays that are very close to the principal axis. They make small angles with the principal axis and are used in ray optics to simplify calculations and predictions about the behavior of light.

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