The physics of blacklights

To even begin to explain how fluorescent lights work, we must start 
with the basics: how light itself is produced.  Light begins at the 
atomic level, when the electrons orbiting the atom have a 
collision with a moving particle.  This causes the electron to 
absorb the energy and jump up to a higher orbital.  This jump 
results in the emission of a light particle, called a photon.  
Light travels in waves, and the wavelength of the light is 
determined by the amount of energy that is released by the 
electron.  Different colors of light are made when different 
atoms are excited.  

This process takes place in all light sources, from the sun 
to the lamps in our dorm rooms to the glowing tubes that light 
our classrooms.  In an incandescent lamp, the atoms are excited 
by heat, whereas in a fluorescent light, the atoms are excited 
by a chemical reaction.  



So, what exactly makes up a fluorescent light? (I know you’re all 
dying to know!) The main component is the sealed glass tube.  
This tube contains a small bit of mercury and an inert gas, usually 
argon.  The argon is kept under very low pressure.  Also in the 
glass tube is some phosphor powder, which coats the inside of 
the glass.  At each end there are electrodes, which are 
connected to an AC current.

OK, now that we know how they’re put together, but what happens 
when you flip the switch?  Well, the first thing that happens is 
the electricity provided by the AC current provides the energy 
to change some of the mercury from a liquid to a gas.  As the 
electrons move through the tube, they collide with the gaseous 
mercury atoms.  These collisions excite the electrons and cause 
photons to be released.

Because of the way the atoms in mercury are arranged, the photons 
that are released through this process are mostly in the 
ultra-violet range. Human eyes can not perceive ultra-violet light, 
so the photons released must be converted into visible light.  
This is why there is a phosphor coating on the glass tube.  A 
phosphor is a substance that emits light when a photon hits it.  
The phosphors absorb the photon given off by the electrons 
of the mercury, and this excites the electrons contained in 
the phosphor.  These electrons then emit another photon, which 
is given off in the form of white light, which is visible, 
unlike the ultra-violet light given off by the mercury.  

Now comes the mechanical part-how all the currents and structures 
in the lamp help make the light work.  We’ll begin with the 
starter switch, which is a component of the old-style fluorescent 
lights and has gone out of fashion.  In this set up, the 
electricity is passed through the starter switch, and causes an 
electrical arc between two electrodes; this causes the ionization 
of the mercury.  The heat from the electricity melts the electrode 
and thus closes the connection and shuts off the switch.  
After a bit, the electrode cools and reopens the connection, 
allowing the light to turn on again.  



The starter switch, as I mentioned before, is old and out dated.  
Now, the most popular kinds of fluorescent lamps are the 
rapid-start lamps.  These lights work in the same basic way 
as the starter-switch lights, but without the actual switch.  The 
new lamps have ballasts, which constantly channel electrodes 
through the electrodes.  The current flow is arranged so that there 
is a different charge at each end of the lamp.   This establishes 
a voltage running through the tube.  The voltage provides an 
electrical arc, which triggers the illumination of the lamp.  

In places where fluorescent lights are used, such as homes and 
schools, the AC current is very strong.  If too strong a current 
passes through the fluorescent light, it can blow out the electrical 
parts of the lamp.  A ballast is used to control the voltage 
passing through the lamp and keep the components from blowing up. 
The most often used types of ballasts are magnetic ballasts.   
They work as an inductor and slow down the changing current.  
However, the alternating current running through the lamp is 
constantly changing direction, so the ballast only has to regulate 
the current in one direction for short amounts of time.  
Ballasts are the reason for the humming and flickering associated 
with fluorescent lights.  The humming is produced when the ballast 
vibrates, and the flickering happens because a magnetic 
ballast operates on a low cycle rate.  

The coolest types of fluorescent lights are black lights.  
Black lights make fluorescent colors “glow in the dark.”  The 
only visible light that a black light can produce is a purplish-blue 
color.  What our eyes can not perceive is all of the ultra-violet 
light that is also coming from the lamp.  What happens when things 
“glow” is basically the same thing that happens in a normal 
fluorescent light.  The black light emits UV light, which is 
absorbed by the fluorescent color, on, say, a poster.  The poster 
will then emit a phosphor in a certain color after absorbing 
the black light.  White tee-shirts and socks seem to glow under a 
black light because of the modern detergents used to clean them.  
The detergents have phosphors in them that allow the clothes that 
they clean to look “whiter than white” in normal sunlight.  
The phosphors in the detergent convert the UV light given off 
by the sun into a kind of fluorescence, which makes the shirt 
appear whiter than white.

Some other substances that glow under a black light include 
quinine, urine, and some paper money.


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