Problem: A microscope is focussed on a black dot. You place a 1cm thick piece of plastic, with an index of refraction of 1.67, over the dot. By how much must you raise the microscope's objective for the dot to be in focus again? Ok, so I'm not sure how to that question at all. Waves and Lights don't seem to click well with me, and it's hard to visualize the problem. I don't even know what formula(s) I would use. Any help with these is really appreciated! I just need to be steered in the right direction Thank you very much!