Method of producing interference of light which was shown by Young in 1801. He used two coherent beams of light that were produced by passing light through a very small circular aperture in one screen, then through two small circular apertures very close together in a second screen. On a third screen, behind the second screen, there will be two overlapping sets of waves and, if the original source is emitting monochromatic light, interference fringes will appear on the third screen (Fig. E8). See coherent sources; interference fringes.