Which discovery allowed humanity to understand that light behaves both as a wave and as a particle?

Wave-particle duality

The concept of wave-particle duality, introduced in the early 20th century, revealed that light and matter possess both wave and particle properties. This discovery overturned the foundations of classical physics, showing that electrons and other subatomic particles could behave as waves under certain conditions. This was demonstrated by experiments such as the double-slit experiment, where individual particles create an interference pattern similar to that of waves. This concept is at the heart of quantum mechanics and has paved the way for new theories and technologies, including quantum computers.