1. The problem statement, all variables and given/known data part1: In the Figure below a lens of focal length 8 cm is placed between a plane mirror and a pin hole. The other side of the pin hole is illuminated by a light source (not shown), so that the light passes through the hole and is then collected by the lens. If the pin hole is at the focal point of the lens, then the light that passes through the lens will be parallel. When the parallel rays hit the plane mirror, the reflected rays will also be parallel, and when they pass through the lens again the rays will be focused at the pin hole. This is called the auto-collimation method for finding the focal length of a lens. For this part, the object is to find the location to place the lens and the pin hole to obtain auto-collimation. Question: Where should the pin hole be placed so that it is at the focal point of the lens? I got part 1 http://img188.imageshack.us/img188/6408/randomlabel.png [Broken] Part 2: The next part of this exercise examines further the concept of focal length. The lens in the last part is used to form the image of the sun. What is the image distance? (Recall that the focal length is 8 cm.) An object is located at the focal point of the lens. The image of the object is located at [.........] (This part is asking for a word, not a number.) 2. Relevant equations 1\o + 1\i = 1\f 3. The attempt at a solution I need help with part B. I don't know where the object is. I assumed it was at the plane mirror therefore the object distance was 4 cm and the focal was 8 cm (given). which was wrong and I'm not sure about the word that they asked for. I tried "focus," "focal length" and "collimation." Why am I wrong?