T spoke up, “The original experiment by Thomas Young in 1803 used sunlight directed by a mirror which entered a darkened room via a pin-hole in a window shutter. A slither of card was held edgewise into the sunbeam. The sunbeam had a diameter slightly greater than the thickness of the card. When the card was placed properly it split the beam into two slivers, one passing on each side of the slip of card. The resulting projection produced a pattern of separated light spots.

Indeed, this experiment led to the wave theory of light in contrast to Isaac Newton’s theory that light is made of tiny bullet-like particles.”

http://www.cavendishscience.org/phys/tyoung/tyoung.htm

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