| Have you ever been startled by a glowing object in the darkness? Let's start by learning about fluorescence and phosphorescence, either of which was likely the source of the light you saw. Materials that emit light after being illuminated with light or electric beams are called fluorescent or phosphorescent materials. Fluorescent material is used in fluorescent lighting and TV cathode-ray tubes, phosphorescent material in wristwatch faces. These materials absorb light and then emit it at a different wavelength. Fluorescence is what occurs when a material absorbs light and then emits it almost immediately, for about 1/1 billion to 1/100,000 second, while phosphorescent materials glow more slowly, for 1/1,000 to 1/10 second. Fluorescence and phosphorescence are also called "luminescence." Electrons on the outermost orbit of atoms and molecules are what emit light as a general rule. Electrons sometimes obtain energy from an external source, causing them to jump onto a higher orbit. When they later descend to their original energy level, known as their "ground" or "normal" state, the difference of energy between two orbital (energy) levels they fall will be released as light. ("Electromagnetic wave" is generally a more accurate description than "light".) The transition to a higher energy level is called "excitation."
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