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If you throw two tennis balls through two open doorways, you get two distinct landing spots on the back wall. But if you shine a laser beam through two microscopic slits, you get a striped pattern of alternating bright and dark bands across the entire room.
This split personalityβ€”behaving like spreading ripples in water under some conditions, yet striking surfaces like tiny localized bullets under othersβ€”is known as wave-particle duality.
How did classical physicists prove light was a wave in the first place?

Historical Wave Model Evidence: Young's Double-Slit Experiment

In 1801, English scientist Thomas Young passed light through two narrow slits to test Isaac Newton's early idea that light was a stream of particles.
In this double slit experiment, light produced alternating bright (constructive) and dark (destructive) interference fringes governed by the path difference equation Ξ”L=dsin⁑θ=mΞ»\Delta L = d\sin\theta = m\lambda
These fringes are governed by the path difference formula Ξ”L=dsin⁑θ=mΞ»\Delta L = d\sin\theta = m\lambda, providing decisive historical wave model evidence.
James Clerk Maxwell later unified electricity and magnetism, demonstrating that light propagates as transverse electromagnetic waves.
Decades later, James Clerk Maxwell unified electricity and magnetism, demonstrating mathematically that light propagates through space as transverse electromagnetic waves.
Wave theory explained reflection, refraction, and diffraction perfectlyβ€”so why did physicists suddenly need particle theory again a century later?

Historical Particle Model Evidence: The Photoelectric Effect

The photoelectric effect occurs when light shining on a clean metal surface knocks electrons completely out of the metal.