Physics Photo of the Week

April 25, 2008

Hummingbird Iridescence
In mid May, 2007, our door was open to enjoy the nice weather, but the screen wasn't up yet.  This hummingbird flew right into the house.  Once birds get inside a house, they have a terrible time finding the way out.  They keep trying the upper windows that are closed.  They refuse to fly low enough to go through the doorway.   After several minutes of vainly flying around beating against the windows, the bird eventually rams itself into a window and stuns itself - falling to the floor.  I was able to pick it up and carry it outside and it eventually regained its composure.  While it was recovering I got some close-up photos. Note the coppery appearance of the bird's back. 

The pigmented color of the bird's feathers are brown, but the iridescence gives it a coppery glow.  The iridesence is caused by diffraction of light by the extremely fine structure in the feathers.  Each feather acts as a diffraction grating - similar to the very fine arrays of pits in a CD disk.  At different angles to the sunlight, different wavelengths of light are diffracted toward the observer. 

The hummingbird soon gathered its strength enough to fly away and continue its remarkable flight - beating its wings about 15 times a second to browse its nourishment from the garden flowers. 

Diffraction gratings are used extensively in many sciences to analyze the wavelengths of light.  This helps to identify chemicals and the materials that exist in stars.  Spectra produced by simple discharge lamps and diffraction gratings have been shown in a previous Physics Photo of the Week (April 16, 2004).  It is amazing that nature provides diffraction gratings very similar to expensive gratings used in analytical instruments!

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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 

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.

Observers are invited to submit digital photos to: