I was solving the below question "An object is 30 cm above a container filled up with water. The lower end of the container is coated silver and acts like an ideal spherical concave mirror of radius 60cm. Find the distance of the image to the surface of water." http://img856.imageshack.us/img856/5575/sadgsdfg.png [Broken] If I find first find the image 1 made by the plan dioptre and than the image 2 that the image 1 makes with the mirror, I find 104cm (that is the correct answer) But if I find first the image 3 that the mirror makes with the object (considering all in air) and THAN the image 4 that the image 3 does when going to water, I find 137cm. Why the answers aren't the same? Shouldn't they be by the reversibility of light principle?