What is the cause of spherical aberration in lenses? State a method to minimise the defect.

Explanation

### Step-by-Step Solution:

1. **Understanding Spherical Aberration**:
- Spherical aberration occurs in lenses when light rays passing through different parts of the lens do not converge at the same point. This is primarily due to the shape of the lens. In a spherical lens, rays that strike the lens near the edges are focused differently than rays that pass through the center.

2. **Focal Length Variation**:
- In a spherical lens, the focal length is not uniform across the entire surface. The central portion of the lens tends to have a longer focal length compared to the edges. This discrepancy leads to a situation where light rays from a point source do not converge at a single point, resulting in a blurred image.

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