A convex lens has a focal length of 15 cm. Determine the focal length and nature of the lens which can be used with the convex lens to obtain a combination with effective power 9 diopter.

Explanation

To solve the problem, we need to determine the focal length and nature of a lens that can be combined with a convex lens of focal length 15 cm to achieve an effective power of 9 diopters.

### Step-by-Step Solution:

1. **Understand the relationship between power and focal length**:
The power \( P \) of a lens is given by the formula:
\[
P = \frac{1}{f}
...