1. The problem statement, all variables and given/known data Hi, I have a question regarding my physics lab assignment. Last week I performed an experiment on the focal length of a biconcave lens using different part (using the radius of the lens) of the lens. This is done by completely covering the lens with a cardboard except for that one spot which radius is to be tested upon So that I obtain different values of focal length for different radius. From my observation it can be seen that the focal length decrease as the radius increase, which is the same as articles which discuss about spherical aberration. However, I'm having trouble finding the formula which relates the behavior or anything that can be inferred from my findings, as this is an open lab class (meaning you can do whatever you want) 2. Relevant equations The Snell's Law: n1sinθ1 = n2sinθ2 3. The attempt at a solution This is the part which really confuses me as my professor said to use the snell's law in order to solve this problem. Also, any advice on how to proceed with the problem? Sorry if the above explanation is not clear as it is hard for me to describe the situation Any help would be greatly appreciated!