Thomas Young Experiment (~1800)
Light from single slit produces coherent light at second screen, where each slit is a single source.
Appearance of “fringes” at a distance screen proved light exhibits both diffraction & interference.
Notes:
Young’s experiment is diagrammed here.
There were two reasons that the wave nature of light hadn’t been discovered previously: 1) The wavelength of light is so small, and 2) most light is “incoherent,” which means that the phases of the oscillations of the direct light from the sources available then were more-or-less random. Today we have lasers, which produce coherent light, but the sources of light available in 1800 were essentially flames.
Thomas produced coherent light by letting nearly monochromatic light fall on a pinhole, then using the light from that pinhole to fall on two other pinholes that were very close together. The angular spacing of the resulting interference fringes increased as the distance between the the pinholes decreased.