What is the formula for mirrors and lenses? My textbook says 1/f = 1/ do + 1/di Then, hi/ho = - di/do Then, M= hi/ho M = di/do Why is that that M equals each other, but one di/do is negative and the other isn't?? Is the formula my in textbook wrong? Also for questions the textbook gives me, it sometimes uses di/do and sometimes uses - di/do. IT'S REALLY CONFUSING ME! Also, can you explain to me all the positive and negatives, that is really confusing, too. Like sometimes numbers will be negative depending on if its a concave mirror or convex or if its a virtual image or what? What's up with that? Can you explain how to do these? I know the answers, but I'm still confused. 1. A student wants to place an object in front of concave mirror to produce an image half the object's size. If the focal length of the mirror is 5cm, how far form the mirror should the object be places? Answer: 15cm This uses the formula M = -di/do when I do it while the next question uses M = di/do. 2. An object place 5cm in front of a concave mirror. The magnification of the object is 2.5. If a real image is produced, what's the radius of curvature of the mirror? (Answer: 16.7) THANKS SO MUCH!