Physics Photo of the Week

Physics Photo of the Week April 13, 2026 Sun Halo - Photo by Donald Collins   Last Wednesday in mid afternoon I noticed the cirrus clouds developing in the blue sky.  I immediately went outside, looked up at the sky - careful not to look at the direct Sun - and saw this magnificent colored solar halo.  These halos are caused by the refraction of sunlight by hexagonal ice crystals suspended in the cirrus clouds high in the atmosphere.  The same effect is often seen as a ring around the Moon.  Not only is the sunlight refracted by the ice crystals, the crystals also disperse (separate) the light into the various colors of the rainbow.  (Note: the photograph was enhanced to emphasize the contrast in colors better than the original cell phone photograph.) The geometry of the hexagonal ice crystals' effects on sunlight is explained in a former PPOW post:  https://physicsphotooftheweek.blogspot.com/2023/09/physics-photo-of-week-september-22-2023.html   ...