Wave-Particle Duality: Light and Matter as Both Waves and Particles

Introduction

Wave-particle duality is the concept in quantum mechanics that every particle or quantum entity may be described as either a particle or a wave. It addresses the inability of classical concepts like "particle" or "wave" to fully describe the behavior of quantum-scale objects.

Historical Milestones

• Einstein and Photons (1905): Albert Einstein explained the Photoelectric Effect by proposing that light is composed of discrete packets of energy, which we now call photons.
• De Broglie Hypothesis (1924): Louis de Broglie proposed that matter also exhibits wave-like behavior, associating a wavelength $\lambda$ to any moving particle: $\lambda = \frac{h}{p}$ where $h$ is Planck's constant and $p$ is the particle's momentum.
• Davisson-Germer Experiment (1927): Conclusively demonstrated the wave-like property of electrons by reflecting them off nickel crystals and observing diffraction patterns.

The Double-Slit Experiment

The double-slit experiment is the classic demonstration of wave-particle duality:

1. When particles (e.g., electrons or photons) are fired one at a time through two slits, they build up an interference pattern on a detector screen behind the slits, characteristic of wave interference.
2. If we place a detector to monitor which slit each particle passes through, the interference pattern disappears, and the particles behave purely like classical bullets.