hello friends, in my course "introductory QM" it says at the end of harmonic oscillator chapter that this may find some applicasions in electromagnetic fields and in crystal physics. now, though i havent covered solid state physics yet, but still i can visualiza the crystal being used as a combination of many oscillators one at each lattice point and any state can be described as superposition of normal modes.is this correct? but i have no idea how oscillators will find applicasions in electromagnetic fields !! i mean, we need some points of stable equilibrium about which a system fluctuates. only then will i get an oscillator quadratic hamiltonian?