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 dewdrop
s 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.