Yesterday on twitter, I ran across a perplexing image: yellow and magenta in a pair of windows that appeared to reflect blue and green, respectively, onto the sidewalk when illuminated by white light from the Sun.
Look at the colors on the wall and reflected / transmitted on the ground! Better than most science museum exhibits pic.twitter.com/vr2zMztCoc
— Kyle Cranmer (@KyleCranmer) July 12, 2016
What is going on?! I decided to put on my “I’m-an-astronomer-who-understands-light” cap and see if I could work it out. The rather circuitous tweet stream begins here, in which I made several errors: I tried to explain a complicated thing before I was fully awake, my brain kept switching key concepts like “absorb” vs. “reflect” and adding vs. subtracting colors of light, and I made a couple assumptions without explicitly stating them (spoiler: they turned out to be incorrect!).
Illustration of how colors of light add together. Source: Wikimedia commons
After much discussion on twitter, I concluded the paper posters in the window were reflecting their true colors (yellow and magenta) in all directions and leaving only their complementary light colors (blue and green) to reflect directly on the sidewalk. If you’re an astronomer, you might ...