Draw a labelled diagram of a common type of bifocal lens. State the function of each of its parts. Name the defect for the correction of which it is used and state its main cause.

Explanation

### Step-by-Step Solution:

1. **Draw the Diagram of a Bifocal Lens**:
- Start by drawing a horizontal line to represent the boundary between the two different lens types.
- Above the line, draw the upper part of the lens as a concave lens (diverging lens).
- Below the line, draw the lower part of the lens as a convex lens (converging lens).
- Label the upper part as "Concave Lens" and the lower part as "Convex Lens".
- Label the horizontal line as "Boundary Line".
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