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   The diagram below shows the path of a ray from the Sun through the cross section of a rod-like hexagonal prism and the similarity to a triangle prism. 

 Rainbows are another phenomenon  of sunlight interacting with another form of water: liquid raindrops.  The geometry of a rainbow and a Sun or Moon halo are entirely different from each other.  Raindrops are transparent liquid spheres whereas the ice crystals responsible for ice halos are solid hexagonal rods similar to wooden pencils.  Rainbows, unlike ice halos, are the result of sunlight reflected from the back sides of spherical raindrops.  Thus rainbows are seen in the direction looking away from the Sun.  Ice halos are seen looking in the general direction of looking toward the Sun, since the ice halos are caused by the sunlight transmitted through the ice crystals.  Always shield the direct view of the Sun when looking at an ice halo!  More details of rainbow physics are explained in the following PPOW post:https://physicsphotooftheweek.blogspot.com/2023/11/physics-photo-of-week.html 

 

  

 

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photo-provider is forbidden.

To join the mailing list, please request to Donald Collins at the e-mail address below.  You will receive no more than one message every two weeks, or when a very interesting physics/astronomy event is happening.

 


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