It was at the Institute that his interest turned mainly to physics. He soon realized that while physicists had generally agreed on major principals in the past, some modern scientists were attempting to disprove outdated theories. Most of Einstein's teachers ignored these new ideas so he was forced to explore on his own. He graduated in 1901, but due to the lack of recommendations from his teachers, he was unable to get a university position. In that same year, he acquired Swiss citizenship. .
In 1902, Einstein secured a position as a clerk in the Swiss Patent Office. This job suited Albert well because it gave him enough time to pursue his own line of research. He developed ideas and published them in specialist journals. Although he was still unknown in the scientific world, he was beginning to attract a large circle of friends and admirers. He married one of those friends, Mileva Meric, in 1903. They eventually divorced, and Einstein married his cousin, Elsa Loewenthal.
In 1905, Albert Einstein published several papers that were of central importance to the development of 20th century physics. In one of these papers, using German physicist Max Planck's quantum theory, Einstein explained the photoelectric effect. The photoelectric effect refers to the emission, or ejection, of electrons from the surface of, usually, a metal in response to incident light. Einstein theorized that light consists of particles (photons), and the energy of such a particle is proportional to the frequency of the light. There is a certain amount of energy (depending on the material) which is necessary to remove an electron from the surface of a material (work function). If the energy of a photon is bigger than this value, the electron can be emitted. In other words, if the energy of the photons is too small, an electron will not have enough energy to escape from the surface, regardless of the number of photons that strike the surface.