What is Fluorescence?

Why do some objects glow under ultraviolet light? In this video, we use fluorescence to explore a question that once shook physics: what is light?

For much of the 19th century, scientists believed light behaved purely as a wave. Thomas Young’s famous double slit experiment seemed to prove it. If that were true, shining enough bright red light onto a material should eventually make it glow, just like blue or UV light. But it does not. No matter how intense the red light becomes, the material refuses to fluoresce.

This puzzle helped Albert Einstein propose a bold idea. Light does not only travel as a wave. It also arrives in tiny packets of energy called photons. Each photon carries energy based on its colour, not its brightness. If a single photon does not have enough energy, it simply cannot do the job.

By looking at a simple green glow, we witness quantum mechanics in action. This episode shows how fluorescence reveals that light behaves as both a wave and a particle, reshaping our understanding of the universe.