1. The problem statement, all variables and given/known data u= distance from the object to the mirror v = distance from the image to the mirror f = focal length I don't understand what they are talking about and I would like to 2. Relevant equations If the image is real 1/u + 1 v = 1/f If the image is virtual 1/u - 1/v = 1/f 3. The attempt at a solution I watched some videos to figure out what they meant by focal length. (My book uses the fact that it is designed to be used in class as an excuse not to bother explaining things). So focal length is the distance between the focus and the mirror. It's how far away from the mirror you have to move an object in order for it to be in focus, whether that be a real in focus image or a virtual one, right? So right now I have my book and there is a mirror 1m away from me. I hold up the book and it is 2m from its virtual image. 1/u - 1/v = 1/f 1m - 1m = 0 Isn't the answer always 0? but that can't be because the focal length isn't always 0. What am I doing wrong / misunderstanding?