Starts With A Bang

Ask Ethan: Why doesn’t Earth’s atmosphere turn sunlight into rainbows? (Synopsis)

The rainbow-like effect seen at right is due to very high-altitude ice crystals affecting the optical phenomenon of a Sun-dog; the Sun itself looks completely white. Image credit: flickr user Kobie Mercury-Clarke under cc-by-2.0.

“It’s a brilliant surface in that sunlight. The horizon seems quite close to you because the curvature is so much more pronounced than here on earth. It’s an interesting place to be. I recommend it.” -Neil Armstrong

There are many impressive optical phenomena that we can see with our own eyes here on Earth. The right configuration of raindrops or ice crystals can produce rainbows, shining light through a prism will separate it into its individual wavelengths, and from high altitudes in the pre-sunrise or post-sunset skies, a full spectrum of colors become visible.

From very high altitudes in the pre-sunrise or post-sunset skies, a spectrum of colors can be seen, but it isn’t due to the same rainbow effects you’re used to. Public domain image.

Yet despite the beauty and similarities of these phenomena, they don’t arise from the same physics. While ice, water, glass or acrylic will slow down light to anywhere from 66% to 76% of its vacuum speed, air itself will only knock it down to 99.97% of its initial value. In fact, you have to look extremely hard — and under ideal conditions — to see any evidence of atmospheric refraction at all.

The rising or setting Sun (or Moon) can produce an image of greener or even bluer light atop it (L) and redder light beneath it (R), due to the minuscule refractive effects of Earth’s atmosphere. Images credit: Mario Cogo (L) and Stefan Seip (R).

Why doesn’t our atmosphere turn sunlight into rainbows? And how does the blue day sky and the red sunset/sunrise sky work? Find out on this week’s Ask Ethan!