Entopic halos if someone looks at a bright outdoor lamp in otherwise dark surroundings, the lamp appears to be surrounded by bright and dark rings (hence halos) that are actually a circular diffraction pattern as in Figure with the central maximum overlapping the direct light from the lamp. The diffraction is produced by structures within the cornea or lens of the eye (hence entopic). If the lamp is monochromatic at wavelength 550 nm and the first dark ring subtends angular diameter 2.5o in the observer's view, what is the (linear) diameter of the structure producing the diffraction?