If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens ? Draw a ray diagram to justify your answer. If the numerical value of power of this lens is 10 D, what is its focal length in cartesian system ?

Explanation

### Step-by-Step Solution:

1. **Identify the Type of Lens**:
- The problem states that the image formed by the lens is always erect and diminished for all positions of the object. This characteristic is typical of a **concave lens**.

2. **Draw the Ray Diagram**:
- To justify that the lens is concave, we can draw a ray diagram:
- Draw a concave lens in the center of the diagram.
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