March 08, 2010
Summary Author: Joe Orman
One of the most commonly seen optical illusions is the highway mirage in which shimmering pools of water seem to cover the roadway far ahead. The effect is caused by a thin layer of hot air just above the ground. The difference in refractive index between the hot air at the road surface and the denser, cooler air above it causes the boundary to act like a mirror: distant objects are reflected. This is categorized as an inferior mirage since the reflected images appear below the real objects. The "water" is actually a reflection of the blue sky, but a close look at this image also shows reflections of a car, power poles, bushes, a mile marker, and roadside grass. Because the reflection occurs solely at very shallow angles, the mirage appears only in the distance and continually recedes as one moves towards it. As you can imagine, this will cause significant frustration to any thirsty desert traveler who believes he or she is actually approaching a water source. This photo was taken with a telephoto lens through the windshield of a moving vehicle -- from the passenger seat. Studying or photographing mirages while driving is definitely not recommended! Photo taken October 10, 2007, on Highway 70 near Alamogordo, New Mexico.