Louis de Broglie (1892 - 1987) went one step further, and made the complementary suggestion that electrons, which had been unequivocally accepted as particles, might have wave like properties, with a wavelength equal to h/p, where 'h' is Planck's constant and 'p' is the momentum. Electrons of a specific energy were diffracted by crystal lattices in just the same way as X-rays. The accepted categorization of the basic elements of the physical world ceased to apply at the atomic level and it was necessary to accept a photon or an electron simply for what it was, defined by its behavior. It was discovered very soon that every kind of physical object that had been labelled as a particle( neutrons, protons and every kind of neutral atom or molecule) also had this wave property, with a wavelength given by de Broglie's formula.