When an atom is excited from one energy state to a higher one by the aborption of a photon, it may return to the lower level in a series of two (or more) jumps if there is an energy level in between. The photons emitted will consequently have lower energy and frequency than the absorbed photon. When the absorbed photon is in the UV and the emitted photons are in hte visible region of the spectrum, this phonomenon is called fluorescence .
Fluorescent objects (phosphors) absorb ultraviolet light and produce visible light.
Fluorescent lightbulbs
work in a two-step process. The applied voltage accelerates electrons
that strike atoms of the gas in the tube and cause them to be excited.
When the excited atoms jump down to their normal levels, they emit
UV photons which strike a fluorescent coating on the inside of the
tube. The light we see is a result of this material fluorescing in response
to the UV light striking it.
Watch:
Fire (torches) | 0.01% |
Candles | 0.02% |
---|---|
Open Lamps (whale oil, kerosene, etc.) | 0.05% |
Lanterns | 0.05% |
Gas Flame Lamps | 0.05% |
Gas Flame Lamps with Mantles | 0.4% |
Carbon Arc Lamps | 2% |
Incandescent Light Bulb | 5% |
Halogen/Quartz Iodide Bulb | 8% |
Fluorescent Tubes (phosphors) | 35% |
Gas Arc Lamps (mercury, sodium,multi-gas, etc.) | 30% - 60% |
Discharge Tubes (neon signs) | 20% - 40% |
LASERS | 5% to 30% |
"Cool" Chemical Light | 2% to 20% |
Light Emitting Diodes (LED's) | 20% to 40% |
Continue to glowlong after the illumination has been removed.
When an atom is raised to a normal excited state, it drops back down within about 10 -8 s. In phosphorescent substances, atoms can be excited by photon absorption to energy levels, which are called metastable and which are states that last much longer, a few seconds or longer. In a collection of such atoms, many of the atoms will descend to the lower state fairly soon, but many will remain in the excited state even for over an hour. Hence light will be emitted even after long periods of time.
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