The nature of fluorescence
Many seemingly mundane objects can emit visible light if exposed to the right conditions. Tonic water, $20 bills, jellyfish, teeth whiteners, laundry detergents — all of these items can manifest the power of fluorescence.
All light consists of photons, which are constantly moving and thus have varying amounts of energy. The undergraduate introductory physics textbook used by Carnegie Mellon, Matter & Interactions, states that only photons within a certain range of energy can be seen by the human eye and are therefore called visible light. Photons with energy levels outside of this range, such as infrared or ultraviolet light, are invisible to the human eye.
Fluorescence is generated when a material absorbs high-energy photons and then emits photons of lower energy. The Optical Microscopy Division of the National High Magnetic Field Laboratory at Florida State University explains that when a fluorescent material absorbs a photon, the excess energy is lost as heat that is subsequently absorbed by neighboring molecules in the material. As a result, photons are re-emitted at lower energies that are now within the range visible to the human eye, making the material appear to glow. Therefore, in order for fluorescent materials to fluoresce, they normally need to be exposed to higher energy light, such as ultraviolet light.
The phenomenon may be easier to understand by looking at a more microscopic picture of what happens to an atom in a fluorescent molecule during light exposure. In an atom, electrons surround a nucleus at various distances. When ultraviolet light shines on a fluorescent material, an electron gets pushed to a higher energy state as it gains energy from the photons. However, electrons have low stability at high energy states, so they almost immediately go back to their original position and re-emit the energy in the form of visible light.
Different types of materials lose different amounts of energy in this process, resulting in different materials fluorescing many different colors. For example, under ultraviolet light $20 bills glow bright green while laundry detergents glow blue.
One of the most familiar applications of fluorescence is the fluorescent light bulb. Inside a common fluorescent light bulb there is a partial vacuum and a small amount of mercury that helps provide the ultraviolet light to initiate fluorescence. The tube is coated with a fluorescent material that absorbs ultraviolet and re-emits visible light. Almost all of the energy in fluorescent lighting goes toward producing light, while the energy in an incandescent bulb goes toward light and heat, so the latter bulb is not as efficient at producing light. A fluorescent bulb can produce between three and seven times more light than an incandescent light bulb.
In many countries, legal banknotes have fluorescent symbols on them that only show under ultraviolet light. The same security features can be applied to identification cards. According to the National Park Service, ultraviolet light can be used to confirm the validity of oil paintings since natural resin varnish layers are fluorescent. People have applied fluorescence to a wide range of fields and will continue tapping its great potential to discover more ways to utilize it in the future.