Hi, http://imageshack.us/photo/my-images/141/optica.png/ The sphere in the picture is made of glass with n = 1.60. The curved side of this sphere is a mirror. The question is why we see two images of the black dot. 2. Relevant equations Snells law? 3. The attempt at a solution One image is created directly by the mirror (1/f = 1/s0 + 1/s1), but I don't know what causes the other image. I don't think the object is one of the images, because we have to calculate the distance to the image (distance object is already given), and i haven't used n = 1.60 in that case. Any ideas?