If you've ever been driving on a summer night, and sworn one of the trees ahead was covered with snow (or otherwise glowing white), it was actually sylvanshine. It is an effect caused by dewdrops reflecting different amounts of light dependent on their angle of contact with leaves. Any angle above 90° is reflective, and at angles greater than about 140° droplets become purely retroreflective, like Scotchlite.

Alistair B. Fraser published a paper documenting the effect in an optics journal in 1994, and the paper was reprinted in Nature the same year. He discovered that only a few trees -- conifers, all -- have needles that are angled correctly to produce the effect, and that the Blue Spruce in particular tended to show sylvanshine strongly. Fraser also noted that a couple of shrubs tend to manifest sylvanshine, notably the Rhododendron and Yew. His research (I'm not making this up, I promise) was done by creeping around wooded areas with a super-bright flashlight in the very late night, and I have a source that says he was at one point detained by a game warden.

Fraser named the effect sylvanshine as a reference to heiligenschein, another optical effect caused by dew.

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