No question in the history of physics has been asked more persistently, or answered more surprisingly, than “what is light?” For four hundred years, the greatest minds in science have proposed theories, conducted experiments, and argued with each other about the fundamental nature of the most familiar phenomenon in the universe. The answers they found along the way did not merely explain light. They reshaped our understanding of matter, energy, space, and time.
This is the story of that question, told through the scientists and the books that changed everything.
The Ancient World: Light as Vision
The earliest theories of light were theories of vision. The Greeks proposed two competing models. Euclid and Ptolemy favored the “emission theory”: the eye sends out rays that strike objects and return information about their shape and color. Aristotle and his followers preferred the “intromission theory”: objects emit something (forms, species, or emanations) that enters the eye.
Neither theory described light as a physical entity with its own properties. Light was understood as a medium of perception, not as a thing in itself. This changed only with the work of the Arab physicist Ibn al-Haytham (Alhazen), whose Book of Optics (c. 1011) established that light travels from objects to the eye, not the other way around. Alhazen’s work, translated into Latin in the 12th century, laid the foundation for all later European optics.
Newton’s Prism: Light Has Color (1666)
The modern study of light begins with Isaac Newton and a glass prism. In 1666, working in his rooms at Trinity College, Cambridge, Newton directed a beam of sunlight through a prism and observed that it separated into a spectrum of colors: red, orange, yellow, green, blue, indigo, and violet. He then used a second prism to recombine the colors back into white light, proving that white light is not pure but is composed of all the colors mixed together.
This was a genuine discovery, and Newton spent the next four decades refining his optical experiments. His book Opticks (1704) presented these experiments in unusually accessible prose and proposed a corpuscular theory of light: light consists of tiny particles that travel in straight lines and interact with matter through forces.
Huygens’ Waves: A Rival Theory (1690)
Fourteen years before Newton published Opticks, the Dutch physicist Christiaan Huygens published his Traité de la Lumière (1690), proposing that light is a wave. His key insight, now called Huygens’ Principle, was that every point on a wavefront acts as a source of secondary wavelets, and the new wavefront is the envelope of all those wavelets.
Huygens’ wave theory explained reflection and refraction with geometric elegance. It correctly predicted that light travels slower in glass than in air (the opposite of Newton’s prediction). And it provided a natural explanation for double refraction in calcite crystals, a phenomenon that Newton’s particle theory handled awkwardly.
But Newton’s authority was overwhelming, and Huygens died in 1695 without followers to champion his ideas. For over a century, the particle theory dominated.
Young and Fresnel: Waves Win (1801 to 1827)
The rehabilitation of the wave theory began in 1801, when Thomas Young performed his double-slit experiment. Light passing through two narrow slits produced an alternating pattern of bright and dark bands on a screen behind them. This interference pattern is the unmistakable signature of wave behavior: where two wave crests coincide, light is bright; where a crest meets a trough, the waves cancel and darkness results.
Augustin Fresnel then built a complete mathematical theory of light as a transverse wave, explaining interference, diffraction, polarization, and rectilinear propagation. In 1850, Léon Foucault measured the speed of light in water and found it slower than in air, exactly as the wave theory predicted. Newton’s corpuscular theory was effectively defeated.
Maxwell: Light Is Electromagnetic (1865)
James Clerk Maxwell unified electricity and magnetism in his famous four equations and showed that electromagnetic waves propagate at the speed of light. The conclusion was inescapable: light is an electromagnetic wave. Heinrich Hertz confirmed this in 1887 by generating and detecting radio waves in his laboratory, proving that electromagnetic waves exist and behave exactly as Maxwell predicted.
By the end of the 19th century, the question seemed settled. Light is a wave, specifically an electromagnetic wave. The debate was over.
Except it was not.
Einstein: Light Is Also Particles (1905)
In 1905, Albert Einstein published a paper on the photoelectric effect showing that light comes in discrete packets of energy, which he called quanta (later named photons). The energy of each photon depends on the light’s frequency, not its intensity. A dim ultraviolet light ejects electrons from a metal surface; a bright red light does not, no matter how intense. This makes no sense if light is a continuous wave, but perfect sense if light consists of individual particles whose energy is proportional to their frequency.
Einstein’s paper, which earned him the Nobel Prize in 1921, brought Newton’s particle theory back from the dead, though in a form Newton would not have recognized. Light is not classical particles or classical waves. It is something new, something that exhibits both properties depending on the experiment.
Quantum Electrodynamics: The Final Theory (1948)
The modern theory of light is quantum electrodynamics (QED), developed by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga in the late 1940s. QED describes light as consisting of photons: quantum objects that are neither classical waves nor classical particles but something more fundamental that can manifest as either, depending on the experimental context.
QED is the most precisely tested theory in all of physics. Its predictions agree with experimental measurements to more than ten decimal places. It explains every known optical phenomenon, from the rainbow to the laser to the behavior of light in gravitational fields. It is, for now, the final answer to the question that Euclid, Newton, and Huygens all tried to answer: what is light?
- Alhazen (c. 1011): light travels from objects to the eye, not the reverse
- Newton (1666/1704): white light is a mixture of colors; light consists of particles
- Huygens (1690): light is a wave; Huygens’ Principle explains reflection and refraction
- Young (1801): double-slit experiment proves light has wave properties
- Fresnel (1815 to 1827): complete mathematical wave theory of light
- Maxwell (1865): light is an electromagnetic wave
- Einstein (1905): light comes in discrete quanta (photons)
- Feynman, Schwinger, Tomonaga (1948): quantum electrodynamics unifies waves and particles
The Books That Defined Our Understanding of Light
Two books stand at the heart of the great light debate, and both are available in beautiful editions from Kronecker Wallis.
Newton’s Opticks features an interactive holographic cover that demonstrates the decomposition of white light into colors, the very phenomenon Newton first observed with his prism. The interior pages are organized by chapter with gradient colors that reflect the science of the spectrum. It is a reading experience that makes the physics tangible.
Huygens’ Treatise on Light is presented in a bilingual French-English edition, with each chapter printed in a different color because the subject of the book is light itself. Hand-sewn with an exposed spine, this edition captures the elegance and clarity of Huygens’ wave theory in a form that honors both the science and the craft of bookmaking.
Together, these two books represent the opposing poles of the greatest debate in the history of physics. Reading them side by side, knowing that both turned out to be partially right, is one of the most rewarding experiences in the history of science.
For those who want to follow the story further, Einstein’s Relativity: The Special and General Theory shows how the nature of light led to the revolutionary ideas that reshaped our understanding of space and time. And the broader story of scientific discovery, from the ancient observers of light to the quantum physicists who finally explained it, is beautifully told through the Portraying Science collection.
A Question That Keeps Getting Deeper
The question “what is light?” has been answered many times, and each answer has turned out to be both right and incomplete. Light is colors (Newton). Light is waves (Huygens, Fresnel). Light is electromagnetic (Maxwell). Light is photons (Einstein). Light is a quantum field (Feynman). Each answer absorbed the previous one and added a new layer of understanding.
Four centuries of asking the same question have produced the most precisely tested theory in physics, a technology that connects the entire planet through fiber optic cables, and a philosophical puzzle about the nature of reality that remains unresolved. The story of light is the story of physics itself: a conversation that never ends, because every answer opens a new question.
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