What is the combined focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm ? What is the nature of the combined lens ?

Explanation

To find the combined focal length of a convex lens and a concave lens in contact, we can use the formula for the equivalent focal length (F_eq) of two lenses in contact:

1. **Identify the focal lengths**:
- Focal length of the convex lens (F1) = +30 cm (positive because it is a convex lens)
- Focal length of the concave lens (F2) = -20 cm (negative because it is a concave lens)

2. **Use the formula for combined focal length**:
The formula for the combined focal length of two lenses in contact is given by:
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