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.
Einstein supported his new theory through his work on the so-called photoelectric effect, which earned him the Nobel Prize in Physics in 1921. First discovered by Heinrich Rudolf Hertz in 1887, the photoelectric effect describes the process by which light causes electrons to be ejected from a material when shone on it.
Now the leading experimental approach researchers use to probe the chemical and electronic properties of materials, photoemission has yielded practical applications for a range of technologies, specifically those that depend on light detection or electron-beam generation, like medical imaging devices and semiconductor manufacturing, among others.
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