What is the focal length to a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore the thickness of the lenses.

Explanation

To find the focal length of a system consisting of a convex lens and a concave lens in contact with each other, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Focal Lengths**:
- The focal length of the convex lens (f1) is given as +30 cm (positive because it is a converging lens).
- The focal length of the concave lens (f2) is given as -20 cm (negative because it is a diverging lens).

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