Initially, Einstein was convinced that light traveled only in particles which he called “quanta”. Bohr's discovery eventually forced Einstein to modify his theory to include the dual nature of light: particles AND waves. It was reported that Einstein never spoke to Bohr again after Bohr published his theory without consulting him first.
Einstein originally believed that all energy (including light) consisted solely of little particles called quanta. However, the Principle of Wave Particle Duality later established that light behaved in two ways: as waves that contained crests and troughs, and as particles – particles that became excited and bumped into each other. The electron gun experiment proved that the behavior of energy from the same source was also consistent – even when shot through separate, different-sized holes or as a single photon. Since then, scientists proved that all matter is composed of some proportion of mass and energy.