When a well-corrected lens is used in place of a pinhole, the geometrical ray divergence is eliminated by the focussing action of the lens, and a much larger aperture may be employed; in that case the diffraction spreading becomes small indeed. The image…

Airy diskformula

…of interference fringes, and the Airy disk, the central spot of light in the diffraction pattern of a point light source, is named for him. In 1854 he used a new method to determine the mean density of Earth. This involved swinging the same pendulum at the top and bottom…