Wave-particle duality is a central idea of modern physics. We now know that light exhibits properties of both waves and particles. Wave theory is the only way to explain interference patterns and diffraction of light but it failed to predict the photoelectric effect discovered in the early 20th century. This was the time when Einstein and Planck introduced the idea of Quanta of light, or Photons. The wave and particle properties of light are inseparable and complementary. One speaks of wave-particle duality, which idea marked the beginning of Quantum mechanics.

ITER (International Thermonuclear Experimental Reactor) is an experimental nuclear fusion project aimed at providing almost unlimited renewable energy. It involves subjecting heavy hydrogen atoms to extreme temperatures and pressure in a confined medium in order to cause fusion of the atoms, thereby reproducing what happens in the stars. The excess energy released during this process can be used to produce electrical power.

An energy level diagram show how atoms can be excited (moved to a higher level energy) or not excited (ground state level energy) . Absorbing or emitting a photon is a common way for an atom to gain or release energy. This emission can be "spontaneous" or "stimulated".

To make a Laser (Light Amplification by Stimulated Emission of Radiation) you have to "pump" energy up from an external source in order to excite a large number of the atoms in the active media, and so promote stimulated emission. You also need a resonant optical cavity in order to force the photons to increase the rate of stimulated emissions. A slightly transparent mirror lets a part of this monochromatic light escape.

Two slits interference pattern created from equal frequency waves passing through the slits.