Light as both wave and particle

Author: Kevin Bu

Editors: Katelyn Ma and Liane Xu

Artists: Athena Mo

The ancient Greeks were the first to think scientifically about what light is and how vision works. Some Greek philosophers, including Plato and Pythagoras, believed that light originated in our eyes and that vision happened when little, invisible probes were sent to gather information about far-away objects. The Arab scientist, Alhazen, on the other hand, proposed that your eyes simply collect the light that falls into them. Alhazen’s theory explained why dark exists, which couldn’t work based on the Greeks’ theory. The idea is that very few objects actually emit their light. The special, light-emitting objects are known as sources of light. However, what exactly is emitted from the sun, and how do we see it? Is it a particle or a wave?

Scientists in the modern era would spend a couple of hundred years seeking for the answer to this question. Issac Newton believed that light is made up of tiny, atom-like particles, which he called corpuscles. Based on this assumption, he was able to explain some properties of light. In the 19th century, long after Newton died, scientists did a series of experiments that clearly showed that light can’t be made up of tiny, atom-like particles. It is acknowledged that two beams of light that cross paths don’t interact with each other at all. If light were made of tiny and solid balls, some of the particles from Beam A would crash into some of them from Beam B, resulting in the collision that would bounce off in random directions, but in reality, the beams of light pass through each other. Light makes interference patterns, which are the complicated undulations that happen when two wave patterns occupy the same space. Only waves make such patterns. As a bonus, understanding that light acts lie a wave leads naturally to an explanation of what color is. In the 20th century, scientists did experiments that show light acting like a particle. For instance, when you shine a light on metal, the light transfers its energy to the atoms in the metal in discrete packets called quanta. Quanta isn’t at all like the tiny and hard spheres Newton imagined. The result that light sometimes acts as a wave and sometimes behaves as a particle led to a revolutionary new physics theory called quantum mechanics.


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