Pick up the dimensionless quantities among the following:
(i) Power of a lens ,(ii) Relative density ,(iii) Average velocity ,(iv) Magnification

Explanation

To determine which of the given quantities are dimensionless, we will analyze each option step by step.

### Step 1: Analyze the Power of a Lens
- **Definition**: The power of a lens (P) is defined as the reciprocal of its focal length (f), i.e., \( P = \frac{1}{f} \).
- **Dimension of Focal Length**: The focal length has the dimension of length, which is denoted as [L].
- **Dimension of Power**: Therefore, the dimension of power is:
\[
[P] = [L^{-1}]
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