The problem is determining the index of refraction. A microscope focuses on a tiny scratch in the upper surface of a plate of glass,then the barrel position is recorded. Next the microscope is lowered by 1.87mm and a focused image of the scratch is seen again. (This is from a reflection off the bottom of the plate) The plate thickness is 1.5mm. What is the index of refraction of the plate of glass? I know the answer is 1.6 but I have tried many differentways.