Define the terms pole, principal axis and centre of curvature with reference to a spherical mirror.

Explanation

To define the terms pole, principal axis, and center of curvature in reference to a spherical mirror, we can break down the definitions as follows:

### Step-by-Step Solution:

1. **Definition of Pole**:
- The pole of a spherical mirror is defined as the **geometrical center** of the mirror's surface. It is the point where the mirror's surface intersects the principal axis.
- In simpler terms, if you imagine a flat line running through the center of the mirror, the point where this line meets the mirror is called the pole.

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