Physics Photo of the Week

Physics Photo of the Week

February 06, 2026  

Shadows on Groundhog Day - Photos by Donald Collins

The weather was was perfect for Groundhog Day last Monday!  Fresh snow had fallen on the weekend, and the Sun was shining with a clear blue sky.  When the groundhog supposedly came out to check on the weather he didn't see his own shadow, but the strange shadow featured in the picture above.  That really scared him so he scurried back into his hole and will be there hibernating for another six weeks - about the time until the spring equinox.  

The groundhog was also spooked by the color of the shadow.  Looking at the color swatches at right, which one (A, B, or C) matches the color of the shadows cast on the white snow on the bright sunny day with the clear blue sky?

 

 

 

 

 

 

If you picked "C" you are correct.  The sunlit parts of the snow receive the direct rays of the Sun.  The Sun emits virtually all colors of the visible spectrum.  The snow which reflects all the colors, thus the snow appears white.  The ice in the snowflakes is transparent, but all the millions of edges of the snowflakes scatter the light in all directions causing the snow-cover in the Sun to appear white - the color of the Sun.  In the shadows, however, the Sun's direct rays are blocked, but the sky is also bright.  The sky appears blue.  The snow in the shadows is not directly illuminated by the Sun's direct rays, but the shadows receive the diffuse light from the blue sky.  Thus the snow in the shadows appears blue.  The high contrast between the bright white snow in the Sun's light and the darker shadows, give the illusion that the shadows are dark grey.

The the light from the sky arises from the myriads of density fluctuations of the air in the sky.  The light scattered from the air is preferentially scattered more of the blue (shorter wavelengths) than the longer wavelength red light.  Since the diffuse blue light from the sky is not blocked by the objects (buildings, people, groundhogs) the the blue light illuminates the shadows where the Sun's rays don't.  Thus the shadows cast on a white background on a clear, cloudless day are blue - the color of the light that penetrates the shadows.  The blue and white swatches were taken from digital "cut-outs" of the main photograph.  The white(A) and the shadow swatch(C) were taken from the rectangular holes in the picture at left that shows the two holes (one in the sunlight, one in the shadow).

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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 photoprovider is forbidden.

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