What you learned in school
Physics textbooks presented light as either a wave phenomenon (like sound) or a stream of particles (like bullets), but never both. Students learned about the great historical debate between wave theory and particle theory, often being taught one perspective as definitively correct while the other was wrong. This either/or approach was fundamental to classical physics education.
What we know now
Physics classes taught that light had to be either a wave (like sound) or a particle (like a bullet) - it couldn't be both. Students learned about the debate between these two theories and were often taught one or the other as correct. Quantum mechanics revealed wave-particle duality: light exhibits both wave and particle properties depending on how it's observed. This fundamental principle applies to all matter and energy at quantum scales. The either/or thinking was replaced by a more nuanced understanding of quantum behavior.