Early in the 20th century, scientists discovered that light has the characteristics of both waves and particles. This revolutionary concept is known as wave-particle duality.

Here's a breakdown:

* Wave nature of light: This was well established by the 19th century through experiments like Young's double-slit experiment, which demonstrated the interference and diffraction patterns characteristic of waves.

* Particle nature of light: This was revealed through the photoelectric effect, where light was observed to eject electrons from metal surfaces, acting as if it was made of tiny packets of energy called photons.

The discovery of wave-particle duality completely changed our understanding of light and has profound implications in physics and quantum mechanics.