Question

Background: Part I: Converging Lenses First, grab the lamp, and place it at one end of...

Background: Part I: Converging Lenses First, grab the lamp, and place it at one end of the optical track. Ensure that the light is directed to shine on the rest of the track (without anything in the way of the "window" on the lamp). Then, select a 5cm converging lens (red tape), and place it in a holster in the middle optical track. Place the arrow in a clip stand, and place that clip stand in a holster in between the lamp and the lens. Similarly place the screen on the other side of the lens. Move the arrow and the screen until they are some distance greater than 20cm apart from each other (say, ~30cm - not too far, or the light won't reach well). Now, we will only move around the lens on the optical track. Shift it around until you see a focused arrow on the screen. We will now take our first set of measurements. Measure the distance from the arrow to the lens holder, then from the lens holder to the screen. (You can do this by looking at the optical track, which has tick marks on it.) Then, take height measurements. Take a distance you can measure on both the object and image (the arrows have plenty of "features" to look at!), and measure that distance on both. Record these as h o and h i (again, with appropriate signs if needed: h o is always positive, but is h i positive or negative here?). If you slide the lens around, you should find another place where the image focuses. Repeat the measurements you did for the previous position with this new focal point.

Question: Why was it important that we placed the screen and the object at least 20cm apart in part I? (Use the thin lens equation, and remember that we have a 5cm converging lens.)