1. The problem statement, all variables and given/known data Three objects P, Q, and R, are viewed through a plane mirror AB. When an obstacle is moved towards the mirror, which object "disappears" (i) first; (ii) last? Diagram. I will describe it in words. The mirror is horizontally placed. Q is placed 10 cm directly above edge A of the mirror. R is placed 20 cm directly above edge A. P is 10 cm (horizontally) to the left of A. 2. Relevant equations None. 3. The attempt at a solution I'm thinking of the positions of the points of incidence of the incident rays coming from the objects. However, when I draw ray diagrams, it doesn't seem so. Maybe the angle of incidence also matters - the smaller, the faster the object "disappears"? I am completely stumped.