While the phosphors in fluorescent lamps are not considered to be toxic, they do contain a tiny amount of mercury. This mercury is an essential part of the operation of the lamp (it is what creates the initial light during the electric discharge). While most fluorescent lamps are simply discarded into landfill, some facilities (including the University of Virginia) dispose of them more carefully. The University of Virginia breaks the lamps to collect the phosphors and then distills the mercury out of the phosphors. The phosphors are then entirely non-hazardous and the mercury is recycled.
While the electric discharge in the tube's mercury vapor emits large amounts of short wavelength ultraviolet light, virtually all of this ultraviolet light is absorbed by the tube's internal phosphor coating and glass envelope. As a result, a fluorescent lamp emits relatively little ultraviolet light. I think that the ultraviolet light level under fluorescent lighting is far less than that of outdoor sunlight.
The exact composition depends on the color type of the bulb, with the most common color types being cool white, warm white, deluxe cool white, and deluxe warm white. In each case, the phosphors are a mixture of crystals that may include: calcium halophosphate, calcium silicate, strontium magnesium phosphate, calcium strontium phosphate, and magnesium fluorogermanate. These crystals contain impurities that allow them to fluoresce visible light. These impurities include: antimony, manganese, tin, and lead.
The starting process erodes the electrodes of a fluorescent tube through a phenomenon called sputtering. A typical fluorescent tube will last about 50,000 hours if left on continuously but only 20,000 hours if it's turn on for just 3 hours at a time. From that tidbit, I think its fair to say that a fluorescent tube can only start about 10,000 times. If the tube costs $5, you are spending about 0.005 cents per start. If the electricity to operate that tube costs about 0.2 cents per hour, then turning the tube off for about 1.5 minutes saves the same amount of money in electricity as it costs in tube life when you turn the tube back on. In short, if you turn the lamp off for less than about 1 minute, you're wasting money. But if you turn it off for more than 10 minutes, you're saving money. In between, it's not so clear. There is a myth that turning on a fluorescent lamp consumes a huge amount of electricity so that you shouldn't turn the lamp off and on. There is simply no basis to that myth.
Light is an electromagnetic wave—an excitation of the electric and magnetic fields that can exist even in "empty" space. Light's electric field creates its magnetic field and its magnetic field creates its electric field and this self-perpetuating arrangement zips off through space at a phenomenal speed—the speed of light. Light is created by moving electric charges, which first excite the electromagnetic fields. Light is also absorbed by electric charges, which obtain energy from the light's electromagnetic fields.
Like everything else in the universe, light exhibits both wave and particle behaviors. When it is traveling through space, light behaves as a wave. That means that its location is generally not well defined and that it can simultaneously pass through more than one opening (the way a water wave can when it encounters a piece of screening). But when light is emitted or absorbed, it behaves as a particle. It's created all at once when it's emitted from a particular location and it disappears all at once when it's absorbed somewhere else. This wave/particle arrangement is true of everything, including objects such as electrons or atoms: while they are traveling unobserved, they behave as waves but when you go looking for them, they behave as particles.
Since plants appear green, they are absorbing mostly the red and blue portions of the visible light spectrum. Blue light is particularly important to them. Incandescent light contains relatively little blue light, so it probably doesn't help plants very much. Because fluorescent lighting provides more blue light than incandescent lighting, fluorescent lighting is certainly better for plants.
There are a few molecules that can be chemically oxidized to produce new molecules that then spontaneously emit light. The chemical reactions that occur in these special molecules leave the resulting new molecules electronically excited—their electrons are in states that have more than the minimum allowed energies. As these energetic electrons subsequently shift to states with less energy, they release some of that energy as light.
In a firefly, the molecule that is being oxidized is called luciferin. It's combined with oxygen and the important biological energy storage molecule ATP (adenosine triphosphate), assisted by a catalyst protein called luciferase. A series of reactions then occurs, culminating in the formation of excited decarboxyketoluciferin. This molecule emits a photon of green light and becomes normal decarboxyketoluciferin.
Luminol, a molecule used in many cold light products, is a somewhat simpler molecule that is much easier to synthesize commercially than is luciferin. When it's oxidized with hydrogen peroxide and potassium ferrocyanide, it forms an excited molecule that emits a photon of blue light. This blue light is often shifted to green or orange with the help of a fluorescent dye. The dye absorbs the blue light and uses its energy to emit green or orange light. This material is commonly used in light sticks and glowing necklaces or toys.
Not unless you will consider a black hole to be a black bulb. For a "bulb" to absorb light that isn't heading toward the bulb, that bulb will have to attract the light toward it. Since light has no electric charge, the only force that the bulb can exert on light is gravitational force. While a black hole's gravity is strong enough to attract and ensnare light, it wouldn't make a very practical bulb. However, it is possible in certain circumstances to add light to previously existing light and, in doing so, create a dark shadow that wasn't present before. This process is called interference, where two light waves cancel one another in a particular region of space and prevent any light from reaching a certain spot. But this cancellation is difficult to achieve, except with lasers, and doesn't occur everywhere in space—the light doesn't vanish, it just gets redistributed. Overall, the idea of a black bulb is just not realistic.
I would guess that the lower wattage tubes will work fine in your existing fixtures, but I am not expert enough to be certain. The 25W tube itself is evidently built so that a smaller current flows through it than through a normal 40W tubes when the two are exposed to similar voltages. The ballast's job is to prevent a catastrophic rise in that current by adjusting the voltage across the tube dynamically during each half cycle of the power line and to keep the tube operating even as a half cycle is coming to an end. Although the 25W tube will draw less current than the ballast expects, the ballast should behave pretty well. I would expect that the tube designers have anticipated this situation and have built the tube to operate with the standard ballast. If a reader knows better, please let me know.
I believe that the glow sticks contain luminol and hydrogen peroxide, which mix when you crack the glass ampoule and begin to emit light. There are several other chemicals present in the sticks to assist and control the process, but the principal reaction is one in which the hydrogen peroxide oxidizes ("burns") the luminol molecule. The result is a product molecule that is initially in an excited state—its electrons have more energy than they need—and it emits a particle of bluish-violet light. Since our eyes aren't particularly sensitive to that bluish-violet light, it's often converted into more visible light with the help of a fluorescent dye. The green light sticks probably contain sodium fluorescein molecules, each of which can absorb a photon of bluish-violet light and reemit some of its energy as a photon of green light. Other dyes, probably rhodamines, are used to make red or orange light sticks.
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