March 8, 1999

Weird shadows --
I've been noticing a brown shadow from my brown chair on my blue rug. (At first I thought it was a stain on the rug.) Yesterday, I noticed my cat had a yellow-brown shadow on a black & white chair. I finally tracked down the diffuse light source that was filling in these otherwise dark shadows (turns out to be light through yellow slats, reflected several times and thus made more diffuse and difficult to locate.)

So, when you're walking the dog on a white surface (especially snow), do you notice that the shadows are sometimes a bit blue? The diffuse light from the sky can fill in the shadows. (It's not bright enough to notice where the sunlight hits directly.)

Are there any other colored shadows that are easy to notice in daily life? (I know the dramatic ones can be produced intentionally.)

Author's Response: 

On snow, if the day is clear and the sky blue, the dog's shadow will have a bluish cast. You can often see bluish skylight on other types of ground as well, such as in the shadows of trees on a clear summer day.

In fact, for this reason, shades of blue are referred to as "cool" colors, whereas shades of yellow and red are called "warm" colors. Spectrally, you'd expect the shorter-wavelength, higher-energy blue photons to be thought of as "hotter." But stand in the yellow sunlight and then in the bluish shadowlight on a hot summer day and you'll know why it's the other way around.