To clear up the confusion of signs to use for mirrors and lenses. Would this be a correct way of memorizing the signs? For the equation: 1/s_o+1/s_i=1/f Mirrors: Concave: Since light actually converges at the focal point, it's positive. Any image that is on the same side as the focal point, object is positive. Any image on the other side is negative. Convex: Since light diverges away from the virtual focal point, it's negative. Any image formed on the same side of the focal point is negative (virtual) and image formed on the other side of the virtual focal point is positive (real). Lenses: Converging: Since light rays actually passes through both of the focal points, they are both positive. images are always positive? Diverging: Since light rays never pass both of the focal points (is this true?) both of the focal points are negative. All images are negative and virtual?