Why Black Lights Make Things Glow

Scott June 10, 2012 3
Today I Found Out why black lights make things glow.

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.

This glow is created by the special way phosphors use the energy from UV light. When a photon from UV light hits the phosphorous material, it causes the electrons to get excited and stray farther from the nucleus than they normally would. When the electron falls back to its normal state, some of the energy is lost in the form of heat. When the UV light wave is reflected back to your human eyes, it now has less energy, therefore a shorter wavelength. This wavelength is in the range that we can “see”. The end result being, you cannot see the majority of the light coming from the source, but you can see its reflection off of objects containing phosphors.

Light is merely electromagnetic radiation. Electromagnetism is one of the four fundamental forces in the universe, the others being gravity* and the nuclear weak and strong forces  This “light” radiation comes in many forms you probably recognize, from the long wavelength (low frequency radio waves, microwaves, and infrared waves), to the shorter wavelength (higher frequency ultraviolet waves, x-ray waves, and Gamma-ray waves). Right in the middle of these is a sliver of the spectrum that we can see with our human eyes, namely visible light waves.

The construction of bulbs that create UV light is fairly simple, though there are several different ways to go about making “black lights”.

A normal Fluorescent bulb creates light by channeling electricity through a conductive inert gas. They add a small amount of mercury inside the tube, or bulb, that will give off light photons when energized. Only a small amount of the light is in the visible spectrum. Most of what is produced is already in the UV range. To get the visible light out of the bulb, a phosphorus coating is applied to the glass and reacts to the UV waves and creates the visible light hospitals everywhere are famous for!

For black-lights, there are two main designs most commonly used. The “tube black light” is very similar in design to a standard Fluorescent bulb. The main difference is in the phosphorus coating. Instead of creating light in the visible spectrum, the coating absorbs harmful 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 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.

So whether you want to throw a “Rave” and know for sure who’s been brushing their teeth, or check your hotel sheets for blood, urine, and semen, black light is a phenomenon that everyone seems to enjoy! So shut off all the visible light sources and let the black light show you what you most likely never wanted to see!

*This is not the venue, but don’t get me started on gravity. It’s not a fundamental force. I personally have spent several years doing research on this very subject, as have numerous others, so I’m not just saying that off the cuff. But considering the owner of this site, Daven, only allows me to publish things here that are hard fact, at least as far as we “know” today, and that I’m sure most here don’t want to wade through pages and pages of mathematical formulas and abstract concepts in an article that’s supposed to be about black lights to see if my supposition holds water, I’ll save my “gravity is not a fundamental force” arguments for more appropriate venues.  Just know that it would not be at all surprising if within our lifetime it is proven conclusively that gravity is not a fundamental force.  When this is proven, I’ll be editing this article with great ebullience. ;-)

Bonus Facts:

  • UV “radiation” and UV “light” are the same thing. Excessive exposure to UV radiation has long been shown to increase your risk for skin cancers, eye damage, and skin aging, while suppressing the immune system and its ability to fight off these problems.  Wear your sun screen and sun glasses kids!  And remember, UV rays are not all filtered out by clouds.  In fact, thanks to a bizarre phenomenon called “cloud enhancement” the level of UV radiation hitting you can actually sometimes be increased by as much as 20%-30% on a cloudy day over a clear one. Exactly what’s going on here to cause this is not yet fully understood, even though it has been a well researched phenomenon since the 1960s.  So bottom line, don’t make the mistake my brother and sister made one fateful day near L.A. when they hung out at the beach all day on a cloudy day, only to then have to drive about 17 hours with extremely severe sun burns over most of their bodies because “the sun isn’t out, so we don’t need sunscreen”.  The same goes for your eyes.  Just because it’s cloudy, doesn’t mean those UV rays aren’t causing damage to your eyes.
  • There are 3 types of wavelengths classified as UV. These are UVA, UVB, and UVC. UVA have the longest wavelengths at 320-400 nanometers (nm). UVB range from 290-320 nm. UVC wavelengths are absorbed by the ozone layer and as such do not reach us here on the Earth surface.
  • Visible light wavelengths range from about 380 nm to about 740 nm.
  • UVA wavelengths account for about 95% of the UV radiation that we are exposed too.
  • UVB radiation was once thought to be the dominant precursor to skin cancers from UV radiation. Scientists thought UVA was not much of a concern. This is because UVA radiation is less intense and it penetrates more deeply than UVB; so it was thought that they did not damage the outermost layer of the skin (epidermis). Numerous recent studies have disproved that theory and UVA waves have been shown to damage skin cells at the basal layer of the epidermis, which is where most skin cancers occur.
  • Ever wonder how tanning booths can make you burn in 10 minutes while it may take hours in sunlight? Tanning booths emit UVA light sometimes as much as 12 times that of what reaches us from the Sun. So the next time you feel like getting a tan via a tanning bed, know that people who use tanning booths are 2.5 times more likely to get squamous cell carcinoma and 1.5 times more likely to develop basal cell carcinoma, neither of which do you want to get! For you teenage girls out there who just love your booth time, you should know that the first session you get in a tanning bed will increase your risk of melanoma by 75%. Yes, that’s just after the first exposure!  Save yourself a lot of potential medical grief and improve how your skin will age by skipping purposefully tanning altogether.  Better to be pale than prematurely wrinkly or, you know, dead from skin cancer.
  • Aside from revealing those around you who have dandruff, black lights have very practical purposes. One of the most helpful is in the form of the “bug zapper”. Bug zappers emit UV light. Insects, unlike humans, have the ability to see UV light. When attracted to the source, they will receive a life-ending shock from whatever electrical mechanism that surrounds the light. One of the reasons humans can see more of the light from a bug zapper, than from any other black light, is that bug zappers use clear glass (because it’s cheaper) instead of filter coated glass. This leaves more visible light available for us to perceive.
  • Another ingenious use for black lights is to diagnose certain types of bacterial infections. 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.
  • The Pseudomonas bacteria aren’t all bad. Bacteria have proteins in their cell walls that will bind with water, even when the microbe itself is dead. They do it so efficiently that it almost mirrors the way ice naturally forms. The end result is that snow and ice can form at warmer temperatures. Some ski resorts have caught on to this little phenomenon and have started adding dead microbes to their artificial snow-making machines. Scientists have also discovered that natural snow contains large amounts of these microbes. Though helpful for the ski resorts, one type, Pseudomonas syringae, is a nuisance for farmers as it almost immediately destroys crops and plants below freezing.
  • There are many types of phosphorus materials that “fluoresce” when exposed to UV light. These materials all tend to have rigid molecular structures that contain delocalized electrons (ones that are not associated with any specific atom within the molecule). Some common examples include: white paper made after 1950 (after 1950 because that’s when manufactures began adding fluorescent compounds to paper, making it appear whiter); petroleum jelly; tonic water (due to the presence of quinine); and the edges of US currency (an added security feature to help prevent counterfeiting), among many others.
  • Some common vitamins that fluoresce include A, and B vitamins, niacin, thiamine, and riboflavin. Antifreeze, tooth whiteners, and chlorophyl also glow bright when exposed to UV light. Most laundry detergents will also fluoresce, which helps your white clothes glow extra bright under a black light. Irish spring body soap and “Mr. Clean” cleaner also contain phosphors. As mentioned, there are several types of body fluids that will glow under UV light, blood, semen, and urine for example.
  • On average in the United States, UVB radiation is most prevalent between 10AM and 4PM from April to October.

Expand for References

Enjoy this article? If so, get our FREE wildly popular Daily Knowledge and Weekly Wrap newsletters:

Subscribe Me To:  | 
Print Friendly
Check Out Our New Book and "Nerdy Stuff" TIFO Shop! »


  1. KO October 19, 2012 at 7:47 pm - Reply

    What on earth do hospitals have to do with fluorescent lights?

    • Daven Hiskey
      Daven Hiskey October 19, 2012 at 9:42 pm - Reply

      @KO: Scott’s a paramedic and so spends a lot of times in and around hospitals, which are typically extremely brightly lit with fluorescent lights.

  2. Melanie November 7, 2013 at 1:38 pm - Reply

    Just a little discrepancy I noticed –
    “When the UV light wave is reflected back to your human eyes, it now has less energy, therefore a shorter wavelength”
    In actuality, by losing energy it emits a longer wavelength. Ultraviolet light has shorter wavelengths than visible light because of its high energy. I hope you don’t mind a little constructive criticism, because otherwise this is a well written and informative article.

Leave A Response »