At the beginning of the 19th century, the nature of light was still hotly debated. Isaac Newton (Article 151) had strongly advocated the corpuscular (particle) theory of light, and his authority largely held sway in the scientific community. The English polymath Thomas Young (1773–1829)—a physician, physicist, and Egyptologist—challenged this view. In 1801, his ingenious Double-Slit Experiment provided undeniable visual evidence that light behaves as a wave, temporarily settling the question of light's fundamental nature.

The particle theory suggested that light was made of tiny balls (corpuscles) ejected from a source. The wave theory, supported earlier by Christiaan Huygens (Article 127), suggested light was a disturbance (wave) traveling through a medium (the theoretical aether).

• The Test: Young recognized that waves exhibit unique phenomena—specifically interference and diffraction—that particles do not. He designed an experiment to test for these characteristics.

Young directed a beam of light through two small, closely spaced openings (slits) onto a screen behind them.

• Particle Prediction: If light consisted of particles, one would expect to see only two bright bands on the screen, directly corresponding to the two slits.

• Wave Prediction: If light consisted of waves, the waves passing through the two slits would act as two new coherent sources. These waves would overlap and interfere with one another—areas where the wave crests overlap would reinforce each other (constructive interference, creating bright light), and areas where a crest met a trough would cancel each other out (destructive interference, creating darkness).

Young's experiment produced the latter result:

• The Finding: Instead of two bright bands, he observed a repeating pattern of alternating bright and dark bands (fringes) across the screen. This was an interference pattern.

• The Conclusion: Interference is a phenomenon unique to waves (like water waves or sound waves). The appearance of the interference pattern provided conclusive evidence that light, whatever its ultimate nature, travels by propagating as a wave.

Young also used the pattern to make the first accurate calculation of the wavelength ($\lambda$) of visible light. By measuring the distance between the slits and the distance between the resulting interference fringes, he could apply wave mathematics to derive $\lambda$. His results showed that visible light has an extremely short wavelength, which helped explain why its wave nature was difficult to observe in everyday life.

Young's experiment settled the debate for over a century, establishing the wave theory as the dominant view, culminating in James Clerk Maxwell's theory of electromagnetism (Article 161).

However, the debate was later dramatically reopened in the 20th century by Planck and Einstein (Articles 153, 49), who proved that light also behaves like a particle (photon), leading to the perplexing concept of wave-particle duality and the subsequent quantum revolution. Modern physics holds that the true nature of light cannot be described fully as either a wave or a particle but as an entity that exhibits both properties, depending on the experimental setup (as further demonstrated by the famous quantum version of the double-slit experiment).

In Conclusion: Thomas Young's Double-Slit Experiment provided the definitive, observable evidence that light behaves as a wave. By demonstrating the characteristic phenomenon of interference (the alternating bright and dark fringes) when light passes through two apertures, Young provided the empirical proof that established the wave model of light, ending centuries of scientific debate and laying the necessary groundwork for the classical understanding of electromagnetism.

At the beginning of the 19th century, the nature of light was still hotly debated. Isaac Newton (Article 151) had strongly advocated the corpuscular (particle) theory of light, and his authority largely held sway in the scientific community. The English polymath Thomas Young (1773–1829)—a physician, physicist, and Egyptologist—challenged this view. In 1801, his ingenious Double-Slit Experiment provided undeniable visual evidence that light behaves as a wave, temporarily settling the question of light's fundamental nature.

The particle theory suggested that light was made of tiny balls (corpuscles) ejected from a source. The wave theory, supported earlier by Christiaan Huygens (Article 127), suggested light was a disturbance (wave) traveling through a medium (the theoretical aether).

• The Test: Young recognized that waves exhibit unique phenomena—specifically interference and diffraction—that particles do not. He designed an experiment to test for these characteristics.

Young directed a beam of light through two small, closely spaced openings (slits) onto a screen behind them.

• Particle Prediction: If light consisted of particles, one would expect to see only two bright bands on the screen, directly corresponding to the two slits.

• Wave Prediction: If light consisted of waves, the waves passing through the two slits would act as two new coherent sources. These waves would overlap and interfere with one another—areas where the wave crests overlap would reinforce each other (constructive interference, creating bright light), and areas where a crest met a trough would cancel each other out (destructive interference, creating darkness).

Young's experiment produced the latter result:

• The Finding: Instead of two bright bands, he observed a repeating pattern of alternating bright and dark bands (fringes) across the screen. This was an interference pattern.

• The Conclusion: Interference is a phenomenon unique to waves (like water waves or sound waves). The appearance of the interference pattern provided conclusive evidence that light, whatever its ultimate nature, travels by propagating as a wave.

Young also used the pattern to make the first accurate calculation of the wavelength ($\lambda$) of visible light. By measuring the distance between the slits and the distance between the resulting interference fringes, he could apply wave mathematics to derive $\lambda$. His results showed that visible light has an extremely short wavelength, which helped explain why its wave nature was difficult to observe in everyday life.

Young's experiment settled the debate for over a century, establishing the wave theory as the dominant view, culminating in James Clerk Maxwell's theory of electromagnetism (Article 161).

However, the debate was later dramatically reopened in the 20th century by Planck and Einstein (Articles 153, 49), who proved that light also behaves like a particle (photon), leading to the perplexing concept of wave-particle duality and the subsequent quantum revolution. Modern physics holds that the true nature of light cannot be described fully as either a wave or a particle but as an entity that exhibits both properties, depending on the experimental setup (as further demonstrated by the famous quantum version of the double-slit experiment).

In Conclusion: Thomas Young's Double-Slit Experiment provided the definitive, observable evidence that light behaves as a wave. By demonstrating the characteristic phenomenon of interference (the alternating bright and dark fringes) when light passes through two apertures, Young provided the empirical proof that established the wave model of light, ending centuries of scientific debate and laying the necessary groundwork for the classical understanding of electromagnetism.