Hello! I'm stuck on this problem... A thin flat plate of partially reflecting glass is a distance (b) from a convex mirror. A point source of light (S) is placed a distance (a) in front of the plate so that its image in the partially reflecting plate coincides with its image in the mirror. If b = 5.20 cm., and the focal length of the mirror is f = - 51.0 cm., find "a" in centimeters. http://img254.imageshack.us/img254/8801/221ch3031p04lt9.png [Broken] I tried solving for p but my answer came out incorrect. 1/p + 1/q = 1/f p = 1/[(1/f)-(1/q)] p = 1/[(1/-51.0 cm)-(1/5.20cm)] = ... Please help me figure out what I'm doing wrong. Thanks in advance!