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!).
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 recognize this as analogous to Kirchhoff’s laws of spectroscopy: a nebula viewed from most lines of sight shows emission lines, but when it’s viewed in front of a bright object instead, you see absorption lines.
Yeah, that was wrong.
I took a closer look at the photo later that day, and realized the rectangles in the window were not posters at all, but looked more like transparent cellophane! That made things easier, I postulated:
Then no need for the directional reflection argument! Y filter absorbs all but Y light, leaving only blue to reflect onto the ground: W-Y=B
— Meredith Rawls (@merrdiff) July 12, 2016
Wrong again, though perhaps a bit closer.
The real explanation appeared today, when the original photographer returned to collect more data.
Like oil on water thin films cause interference that depends on wavelength & angle. Sidewalk & I saw different views pic.twitter.com/5PoI087fvC
— Kyle Cranmer (@KyleCranmer) July 13, 2016
@KyleCranmer mine was a complicated mix of transmission, reflection, etc, but sidewalk was all reflection pic.twitter.com/ZFOY8acVr8
— Kyle Cranmer (@KyleCranmer) July 13, 2016
@KyleCranmer yea! ☀️popped out 🌈. Here’s a good view pic.twitter.com/ykKNqIyyWZ
— Kyle Cranmer (@KyleCranmer) July 13, 2016
Trust me, you definitely want to play all those short videos (no sound required). The viewing angle changes everything! THAT’S SO COOL! Have you ever wondered why the cry of the scientist is “more data!”? It turns out that viewing something from more than one perspective can be very instructive, or should I say… illuminating? It’s enough to make this astronomer wish we had a way to fly halfway across the galaxy with a fleet of telescopes. Alas, space is way too big for that.
So there you have it: a learning experience, a more nuanced understanding of color, and a scientific quest all rolled into a handful of tweets.
AstronoMerrdiff 