# Rainbow reflections from salt crystals

Especially on a sunny day in the winter or spring, I often notice faint rainbows when I look down at the edge of the road by the curb. I am not sure what causes these rainbows, but I believe that they are caused by light reflecting off tiny salt crystals that have accumulated on the ground.

Why do rainbows form under such conditions? The standard mathematical explanation for rainbows assumes spherical raindrops for simplicity, and for water droplets suspended in air, that seems like a reasonable approximation. But salt crystals (or whatever particles on the ground that are causing the rainbows) have irregular shapes and are dispersed randomly on the ground. Wouldn't such random configurations preclude a reflection pattern as coherent as a rainbow?

EDIT: To help clarify what I'm talking about, here is a short video of the type of rainbow effect that I have observed. (You might want to turn off the sound; unfortunately I don't have a video editing program to remove the sound.)

EDIT: The accepted answer below mentions retroreflector glass beads. I took a sample from the ground, which a friend of mine took a close-up photo of as well as a flash photo of. Both photos support the glass bead theory.

• Or thin films of oil make nice rainbow effects. You have assumed a cause that doesn't seem to necessarily apply. Commented Apr 20, 2022 at 15:52
• @JonCuster It's true that my assumption may be incorrect. On the other hand, when I observe this phenomenon, it's when the ground is bone dry and the weather is bright and sunny. (Oil is certainly not the cause; those color patterns are swirly and not band-shaped or bow-shaped, as these patterns are). If I move my head side to side, it creates an illusion that the rainbow is a few inches above the ground, and moving as I move. Commented Apr 20, 2022 at 17:19