Wave - particle
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.