Missed this part of the lab and I need to complete my lab write up. We have a light source at one end of a track, and at the other end a screen. there is a positive lens in between them. When we set the screen at a distance away from the light source, lets say one meter, and slide the lens on the track connecting them, starting from the light source and moving towards the screen, at two distinct locations we get a focussed image. The first image, closest to the light source, is roughly 4X the size actual size of the image, while other is roughly 1/4X the size. My question is, why is this? what are those lengths? are the the focal length? and how can I show these two lengths mathematically? Thanks!