May 06, 2004
When some substances are exposed to invisible ultraviolet light they emit visible light. This phenomenon is called fluorescence. The ultraviolet light interacts with the electrons in the atoms of a fluorescent substance, and some of the electrons move briefly to higher energy orbits and then quickly drop back to their original orbits. In dropping back, an electron loses energy and gives off visible light.
The top photo shown above was taken in a cave near Akron, New York by white or visible light generated by the flash unit of my camera. Several small stalactites have started to form under the overhang --the longest is roughly an inch (2.5 cm) in length. These formations are made of calcium carbonate (calcite). The bottom photo is of the same area and was taken by using two longwave ultraviolet lights to illuminate the area. This type of light is commonly known as "blacklight" and is widely used in entertainment and advertising. I used a digital camera and an exposure time of four seconds.
When the ultraviolet lights were turned off, a brief (two to three seconds) greenish-white phosphorescence of the formations was noticed. The phosphorescence is caused by the energized electrons slowly returning to their normal orbits.