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The Double-Slit Experiment

By the early 19th century, most physicists agree with Newton's well-established theory of light, which says that light takes the form of particles—what we now call photons. But English scientist Thomas Young isn't convinced, and in 1803, he designs an experiment to test the status quo. Young aims a beam of light at a barrier that has two slits. If light is made of particles, he reasons, those particles should travel in a straight line through the slits, projecting two distinct lines of light on the screen beyond the barrier. Instead, Young sees a series of dark and bright lines on the screen, a pattern that could only be produced by waves of light interfering with each other. And yet other experiments, both before and after Young's, convincingly show the particle nature of light. Physicists are left with the unsettling conclusion that light—and, as they later find, electrons (matter)—has a dual nature: Sometimes it takes the form of particles and sometimes the form of waves.

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