This document discusses the wave-particle duality of light. It describes early theories that light was either a wave (wave theory) or particle (particle theory). Later, Max Planck proposed that light has both wave-like and particle-like properties, exhibiting wave-like interference and diffraction patterns but also behaving as discrete packets of energy (photons) when interacting with matter. Albert Einstein further developed this idea by hypothesizing that the photoelectric effect was caused by photons of light energizing electrons. The double slit experiment also demonstrates light's dual nature by showing it can form interference patterns like a wave even when passing through the slits one photon at a time.