Diffraction of Light
In 1665, Grimaldi observed that the shadow cast by thin wire is much smaller than expected. Consider 2 points A and B on the same wave font A &B is also said to be 2 coherent secondary sources of light wave. You will expect the two to have an interference pattern on the screen, provided the slot is small compared to the wavelength of light. The geometrical pattern is long on the screen. Shows that they exist from a geometrical pattern of attractive dark and bright band after M&N.