The nature of light has long been debated. During the seventeenth century, two rival theories dominated scientific thought. Isaac Newton proposed that light consisted of tiny particles, or corpuscles, travelling in straight lines. In contrast, the Dutch scientist Christiaan Huygens suggested that light behaves as a wave. Huygens’ theory, published in Traité de la Lumière (1690), proposed that every point on a wavefront acts as a source of new waves spreading outward in all directions. By applying this principle, known today as the Huygens–Fresnel principle, he successfully explained reflection and refraction, phenomena that were difficult to reconcile using Newton’s particle model.
Although Huygens’ idea provided a consistent mathematical description of wave behaviour, it lacked direct experimental proof at the time. Newton’s authority and the success of his mechanics ensured that the corpuscular view of light dominated scientific thinking for more than a century. The debate remained unresolved until the early nineteenth century, when the English physicist Thomas Young performed an experiment that transformed the understanding of light.
In 1801, Young passed light through two narrow slits and projected the result onto a screen. Instead of forming two bright bands as would be expected from particles, the light produced a series of alternating bright and dark fringes — an interference pattern characteristic of waves. The bright regions occurred where the waves from each slit reinforced one another, and the dark regions where they cancelled out. This simple experiment demonstrated conclusively that light exhibits wave-like behaviour.
Young’s findings confirmed Huygens’ hypothesis and marked a turning point in physics. The wave theory of light explained a wide range of optical phenomena, including diffraction and interference, and it formed the basis for James Clerk Maxwell’s later description of light as an electromagnetic wave. Yet the wave theory would itself be challenged a century later, when quantum mechanics revealed that light also behaves as a stream of particles, called photons. Together, these discoveries led to the modern understanding of light’s dual nature — both wave and particle — at the heart of quantum physics.