Geoff Chester

Why do the bike paths of Virginia glow like rainbows pouring from the earth?

Geoff Chester was biking in Arlington, Virginia, last weekend when he found himself trapped in a field of iridescent light. A rainbow-hued halo sprung from the bike path underneath him and seemed to latch on, following the cyclist for miles over the Washington & Old Dominion trail like a highly individualistic Rapture.

“I looked down at the ground and saw my shadow and this halo around it,” he says. “I was like, 'I know I'm a good guy, but....'”

Chester doesn't deal in superstitions – he's a public affairs officer at the U.S. Naval Observatory Public Affairs Office in Washington, D.C. So what he did was snap a picture of the glowing bike path and forward it to Tony Phillips, a NASA scientist who runs the engrossing site Space Weather.

Phillips for his part contacted an atmospheric-optics expert across the ocean, Les Cowley, who studies light effects created by airborne dust, water, ice crystals and such, and the men started going back and forth over what could be causing the wacky earthbow.

"The conditions were dry, so it wasn't anything to do with dew,” says Chester. "And it was only visible on the asphalt, not on the grass or a concrete wall when I was looking at my shadow there."

That left only the path as the source for this wondrous light show. Chester and friends developed two hypotheses for how it's happening. The first is that Arlington County sprayed the middle line with some kind of glass-based coating that refracts light in a microcosmic ring, kind of like a majestic 22-degree halo but at ankle level. Here's Cowley explaining on Space Weather how a "glass bead bow" can form:

"Crews marking paint lines on roads often scatter small glass beads onto the paint. The glass beads retro-reflect light and this enhances the visibility of the markings at night. The glass beads – if sufficiently spherical – also produce rainbows. The difference is that the refractive index of glass is greater than that of water and the bow is only about 21° in radius compared to the rainbow's 42°. The glow around the shadow of Geoff's head is an antisolar point phenomenon... produced by refraction through the glass spheres."

The other conclusion is that particles of highly refractive material – perhaps quartz crystals – have blown over from a nearby road bed and settled into tiny fissures in the path, causing the weird illumination. Chester plans to go out on Saturday to take a little sample from the ground to study it under the microscope.

“It's piqued the nerd in me,” he says, “to find out what's going on.”

(UPDATE DEC. 3: Chester says "the mystery is definitively solved! As the first comment poster suggested, the culprits are indeed glass beads. They were spread quite liberally on the trail, filling the spaces between the aggregate pebbles in the asphalt.  I collected some during my ride this past weekend... perfect little spheres of glass." So that's that, then.)

Top photo courtesy of Geoff Chester.

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