is potential energy and kinetic energy real or is it theoretical and is used to explain how thing work? if it is real than where is potential energy stored? Because energy has to be stored somewhere, like in a plant the energy of the sun is stored in its leaves.

Amichelson interferometer operating at a 600 nm wavelength has a 3.02-cm-long glass cell in one arm. to begin, the air is pumped out of the cell and mirror m2 is adjusted to produce a bright spot at the center of the interference pattern. then a valve is opened and air is slowly admitted into the cell. the index of refraction of air at 1.00 atm pressure is 1.00028.how many bright-dark-bright fringe shifts are observed as the cell fills with air?