Category: fluorescent lamps

How are luminol and fireflies related? — JH, Minneapolis, MN

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.

Is it possible to make a black bulb that absorbs light rather than emitting it? — KD, Pflugerville, TX

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.

Exactly what is light? Is it a wave or particles? — MW, Catoosa, OK

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.

What effects do fluorescent lamps have on household plants? — SN, Milwaukee, WI

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.

How much life is consumed each time you turn on a fluorescent lamp? — BL, San Jose, CA

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.

Do regular fluorescent lights emit ultraviolet light? If so, how does the ultraviolet level compare to what we would receive if we were outside? — GF, Barstow, CA

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.

What is the composition of the phosphors used in fluorescent light bulbs? – M

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.

Are there any risks, other than a case of implosion, with regards to exposure to normal fluorescent lighting? – RR

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.

How do light sticks work? – AE

When you bend a plastic light stick, you break a small glass ampoule and allow two chemicals that are contained inside the stick to mix. One of these chemicals is a powerful oxidizing agent and the other is a chemical that when oxidized (“burned”) is left in an electronically excited state. In other words, the chemical reaction between the molecules of the two chemicals creates a new molecule that has excess energy in it. The molecule releases this energy as a particle of light, a photon. Although I am not certain exactly which chemicals are used in a modern light stick, I believe that one is hydrogen peroxide (the oxidizer) and the other is luminol (the chemical that is oxidized). Upon oxidization, luminol emits a photon of blue or ultraviolet light. The green light that you see emerging from a typical light stick is actually a second photon that is emitted by a fluorescent dye contained in the light stick. This dye absorbs the blue or ultraviolet photon emitted by the luminol and then reemits a new photon with somewhat less energy and a green color.

What’s the difference between fluorescent, phosphorescent, and triboluminescent? – DS

Fluorescence is the prompt emission of light from an atom, molecule, or solid that has extra energy. For example, when some of the dyes used in modern swimwear and clothing are exposed to ultraviolet light, they absorb the light energy and promptly reemit part of that energy as visible light—typically brilliant greens and oranges. In contrast, phosphorescence is the delayed emission of light by an atom, molecule, or solid that has extra energy. Glow-in-the-dark objects are phosphorescent—they are able to store the extra energy they obtain during exposure to light for remarkably long times before they finally release that stored energy as visible light. Systems that exhibit phosphorescence rather than fluorescent are those that have special high-energy states that have enormous difficulty radiating away energy as light. Finally, triboluminescence is the emission of light from a surface experiencing sliding friction. Since sliding friction introduces energy into the surfaces that are sliding across one another, it’s possible for that energy to be emitted as light.