In may experimental set-ups the source and screen are fixed at a distance say `D` and the lens is movable. Show that there are two positions for the lens for which an image is formed on the screen. Find the distance between these points and the ratio of the image sizes for these two points.

Explanation

To solve the problem, we will follow these steps:

### Step 1: Understanding the Setup
In the given experimental setup, we have a fixed distance \( D \) between the source and the screen. The lens can be moved, and we need to find two positions of the lens where an image is formed on the screen.

### Step 2: Defining Variables
Let:
- \( u \) = object distance (distance from the lens to the object)
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