1. The problem statement, all variables and given/known data A concave mirror of focal length 10cm in air is put into a beaker containing a liquid up to 7.5cm. Refractive index of the liquid is 1.2. What is the focal length of the mirror inside the beaker? 3. The attempt at a solution I tried proportion method: Depth Focal length 7.5cm 10cm 6.25cm(Apparent depth) 8.33cm But I'm not convinced that this method is a legal operation. Any other way I could find the new f for the mirror?