What exactly is light—and what is it made of? It’s an age-old question that dates back to antiquity, and one of the most important investigations undertaken by scientists looking to understand the nature of reality.

The question of what comprises light—a form of energy that, as it bounces off of objects, allows us to see the world—has led to such spirited debate and discussion in the scientific community that it gave birth to a whole new field: quantum mechanics.

Underlying the debate about the nature of light is yet another mystery. That is, does light behave like a wave, or a particle? When Albert Einstein in the early 20th century proposed that light is both particulate in nature (containing small particles called photons) and wave-like, many were satisfied, if slightly uneasy, about his findings.

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Photo by Ruby Wallau/Northeastern University