Comprised of a sealed tube coated on the inside with a phosphor powder and filled with (usually)
argon and a tiny bit of mercury, the way in which fluorescent bulbs produce light is extremely
fascinating.
While the underlying physical mechanisms in the fixture and bulb can vary a bit in design,
in a nutshell a fluorescent light works via two electrodes at either end of the tube emitting
electrons as they heat up.
Eventually, an arc is created (via varying mechanisms depending on the fixture and bulb
design) with the electrons shooting across the bulb through the ionized gas from one
electrode to the other; as they move through the tube, the tiny bit of mercury in the bulb
is vaporized and as the electrons collide with its atoms, it excites the electrons in
the atoms to higher energy levels.
However, this higher energy level is unstable and post-collision, as the electrons rapidly
return to their original energy level, they release photons, albeit mostly ultraviolet
(UV) light that we humans cannot see.
Not a very useful light source at this point!
The photons in the UV light, however, in turn excite the electrons in the phosphor that
is coating the bulb, similarly causing them to move away from their nuclei to a higher
state; the phosphor electrons then quickly return to their original state, at which point
they also release energy in the form of photons, but this time mostly in the human visible
spectrum, creating the bulk of the visible light that makes these bulbs useful in office
buildings the world over.
Now to the buzzing- Absent preventative measures, the current in the fluorescent tube would
rise to dangerous levels owing to the fact that the electrical resistance of the ionized
gas in the tube drops progressively as it heats up.
So without something put in place to stop the current rising too much, this would be
a cascading problem.
Eventually, it could well flip your circuit breaker or the bulb could explode.
Whatever the case, your lights would quickly stop working.
To manage that, fluorescent bulb fixtures are equipped with a ballast of some sort.
This ballast classically comes in the form of an iron core wrapped in copper wire.
The result is a device that slows the current’s growth, keeping it at a safe level for the
bulb to operate efficiently.
Important to the discussion of buzzing is that the electricity energizes the ballast
into producing a magnetic field.
This is, in fact, how this type of ballast works in the first place- as more current
is passed through, the magnetic field gets bigger, opposing the change in the current
flow and thus slowing its growth enough for the alternating current (AC) to switch directions,
with it dropping to zero and going back up in the opposite direction in the process.
Standard alternating current rates are usually 60 Hz a.k.a. 60 cycles per second (such as
in the United States) or 50 Hz (such as in the U.K.).
Aptly named, for half of an AC cycle, the current’s charge moves in one direction,
and for the second half, the charge moves in the other.
All of this switching back and forth further affects the ballast’s electromagnetic field,
since each time the current switches direction (every half cycle), the polarity of the magnet
also switches; so, since the electromagnet is fluctuating at twice the frequency of the
AC current, its flicker rate is at 100 Hz or 120 Hz, depending on what country you’re
in.
During these fluctuates in magnetic field, the ballast’s core is physically squeezed
and released in a process called magnetostriction that, at a frequency of either 100 Hz or 120
Hz, produces the infamous buzz.
Of course, not all fluorescent bulb fixtures’ buzzes are alike, and this is due to differences
in the fixture’s ballast type, size, how the ballast is mounted, fixture design and
the degree to which the ceilings, walls, sheet metal, etc. muffle or amplify the sound.
Such a common nuisance, the industry publishes “sound ratings” for ballasts, and even
recommends in which settings different ratings are appropriate.
For example, those ballasts with an “A” rating are the quietest (20-24 decibels),
and they are recommended for libraries, churches, reception areas and in TV and radio stations,
while only a “C” level (31-36 decibels) is recommended for a “general office area,”
and retail stores can get by with “D” ratings (37-42 decibels).
For those who hate the buzzing (or perhaps have issues with migraine headaches being
induced by the fluorescent bulbs and their flickering light), electronic ballast (as
opposed to the old school magnetic ones) are available and are even quite common today,
such as generally found in Compact Fluorescent bulbs (CFLs).
These ballasts typically operate at a drastically higher rate than 100 Hz or 120 Hz, usually
in excess of 20,000 Hz.
It should be noted, though, that if you switch to one of these electronic ballasts in your
older fluorescent light fixture (something that is surprisingly cheap to do), you do
need to swap out your fluorescent bulbs to a variety that is rated to work with your
new ballast.
Bonus Fact: Ever wonder why black lights make certain
things glow?
Well, wonder no more: Black lights are not that different from any other type of light,
whether incandescent, fluorescent, or just the age old candle flame.
The difference is that black lights emit most of their light waves just outside the range
humans can perceive, in the Ultraviolet (UV) part of the spectrum.
When a UV light wave hits an object containing substances known as phosphors, those phosphors
will naturally fluoresce and glow as previously described with how fluorescent lights convert
UV light to light in the human visible spectrum via phosphor coating the bulb.
As you might have guessed from this, the construction of bulbs that create UV light is fairly simple,
though there are several different ways to go about making "black lights", with two main
designs most commonly used.
The "tube black light" is very similar in design to a standard Fluorescent bulb previously
described.
The main difference is in the phosphorus coating.
Instead of creating light in the visible spectrum, the coating absorbs UV-B and UV-C light waves
and creates UV-A waves.
"Black" glass tubes will naturally block out the small amount of visible light created
by the energized mercury so that most of what comes through is UV light and a small amount
of visible wavelengths of light that are the closest to the UV spectrum, those being shades
of dark blue and violet.
This is why black lights appear to be a shade of purple to the naked eye.
The second type of common black-light is and incandescent black light bulb.
These work more or less the same as any old-style standard household incandescent bulb.
The difference between the two is the filter that is added to the bulb.
All of the light that is created by the heated filament is filtered out, except those waves
in the infrared and UV-A spectrum.
The filter absorbs the rest, which is why this type of black light tends to get extremely
hot, even for an incandescent bulb, and has a short lifespan.
Incidentally, beyond looking for blood, urine, and semen on your hotel sheets, another ingenious
use for black lights is to diagnose certain types of bacterial infections.
You see, some bacteria naturally fluoresce under UV light and, as such, certain infections
can be diagnosed by shining a black light on the patient.
One such type of infection is caused by bacteria known as Pseudomonas.
This type of bacteria has 191 known species and is the second leading cause of infections
in hospitals.