1. Energy levels and transitions

1.1 Wave-particle duality

The goal of this section is not to answer in detail the complex question “What is light?”. It is quite difficult to speak about the laser without evoking the problem of the nature of radiation. Does it consist of a wave or of particles? The debate had already roused the minds of the most eminent Greek philosophers. (Aristotle “felt a strong inclination toward” a wave nature, Empedocles and Pythagoras, the particle character). After centuries of reflection, research and... controversy, modern science answers with a statement which can seem ambiguous but which in fact sums up rather nicely the state of our knowledge: light behaves sometimes like a wave, and sometimes like particles.