How are electromagnetic waves generated?

When electromagnetic waves are generated or absorbed changes take place in atoms or in the nuclei of atoms. E.g. in lithium when atoms are heated an electron will move to a higher energy level, and when it returns to its original energy level it will generate an electromagnetic wave. Changes made to the nucleus can also produce other electromagnetic waves e.g. gamma rays. Gamma rays can be emitted from the nucleus of radioactive atoms. This causes the nucleus to have less energy than it initially had.