Light can apparently behave like a particle, or like a wave? But under certain circumstances, it must somehow behave like both at once.
One of the first experiments of this sort was described by Hellmuth, Walther, Zajunc and Schleich, in Physical Review A, vol 35, p. 2532 (1987). The scanned results and conclusion from that paper are shown below. A slightly earlier paper by Alley et al., in Proceedings of the International Symposium on Foundations of Quantum Mechanics, (Tokyo, 1986), describes a very similar experiment, but is much harder to find.
You can read a short essay on the puzzle of this duality by John Archibald Wheeler.
Copyright © Michael Richmond. This work is licensed under a Creative Commons License.