Physics Photo of the Week

November 8, 2013

Impressive Rainbow - Photo by Julie Lehman
This brilliant rainbow appeared near Warren Wilson College on the morning of Oct. 23, 2013.  It is ironic for me because I live right beneath it and I completely missed it!  Many thanks to Julie Lehman who saw it and photographed it with her camera phone while briefly stopping to admire the view.  Click on the photo for a larger image.

This rainbow was unusual because it appeared in the early morning soon after the Sun had risen.  We know that not only from the time stamps on Julie's photo but also because I am quite familiar with this scene and it is looking west - away from the rising Sun.  This is also the most brilliant rainbow that I have featured in Physics Photo of the Week.  Thank you Julie for being extra alert with your camera phone!

Rainbows are seen most often in the late afternoon close to sunset.  That is the time when brief thunderstorms frequently occur, and give a chance for the Sun to appear illuminating the distant raindrops from the rain shower.  The afternoon rainbows appear in the east opposite the Sun setting in the West.  Morning rainbows appear in the west opposite the rising Sun in the east as this one happened.  We missed it from our house because our windows face the east.

This rainbow is especially brilliant showing much brighter sky underneath the rainbow than outside the main bow.  This is due to the optics of sunlight through the raindrops.  For the angles for the drops underneath the bow, the sunlight enters near the center of the raindrop and emerges from from near the center of the raindrop, partially reflected by the rear surface of the raindrop.  However for raindrops at a critical angle to the Sun, light enters near one edge of the drop and emerges near the other edge of the raindrop with the light separated into the component colors.  This is the rainbow.  For drops outside the main bow, there is no reflection, hence the sky is darker.  This darker region of the sky outside the main bow is called "Alexander's Dark Band".

If you look closely at the bright rainbow, you can see what looks like interference fringes just beneath the main bow.  These are interference of the light waves called "supernumerary bows".  Immediately underneath the main bow, the light emerging from the raindrop appears as two point light sources very close to each other in each raindrop.  See the PPOW for October 29, 2010 for a laboratory simulation of a large raindrop.  The two apparent light sources interfere with each other - due to the wave nature of light - and cause fringes called Young's Interference.

Another rainbow Physics Photo appeared on April 4, 2008 complete with a ray tracing image.


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. 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 photoprovider is forbidden.

Click here to see the Physics Photo of the Week Archive.

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