Here’s another rule of thumb: If you have two light sources of different colors shining on the same form, the cast shadow from each light source will be the color of the other source.That’s because each colored light shines into the shadow of the other. So when the amber light shines on the cylinder, it casts a shadow that is filled only by the blue light—hence the blue shadow.
Let’s make the scenario even more challenging: What happens to the shadows when you have three colored sources? Here’s the effect from blue, red, and green lights.
Note that the cast shadows behind my raised hand and my head are magenta, cyan, and yellow. The yellow shadow on the far right is cast by the blue light. It’s yellow because it is lit by a mixture of green and red. In the realm of light, as opposed to pigment, green and red mix to make yellow.The bright cyan shadow is cast by the central red light. It’s cyan because it contains the blend of blue and green light. Blue and green light mix to make cyan.
If you’re intrigued by colored light interactions, I’d recommend going to the home improvement or theater lighting store and picking up red, green, and blue lightbulbs that you can put in three different light sockets. The best way to learn this stuff is to set them up and try all sorts of combinations. I took the photo in the New York Hall of Science, which has an interactive setup installed.
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Related GJ post:
Colored Light and Form
