Can we use our fingers to monitor the intensity of ultraviolet light?
The purpose of this science project is to learn whether I can use my fingernails to monitor the intensity of ultraviolet (UV) light.
(Okay, the REAL purpose of this endeavor is to demonstrate the steps of a science project that follows the scientific method. Also, as a result of a bet I lost, I was honor-bound to paint my nails; and if a thing's worth doing, it's worth doing right.)
- Color-changing UV nail polish
- Clear "top coat" nail polish
- Electronic UV light meter
All 10 fingernails will be coated with nail polish that changes color in UV light. Eight fingers will be given additional layers of clear, UV-blocking "top coat." All fingernails will be exposed to sunlight, and the number of fingers that change color will be recorded (along with the intensity of UV light recorded by a meter).
- After coating each fingernail in a different amount of "top coat" nail polish, the number of fingernails that change color will correspond to the intensity of UV light.
- Using an electronic UV light meter, I can calibrate the number of color-changing fingers to the UV Index scale, turning my hands into UV Index meters.
Let's get to science-ing!
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Step 1: Find Some UV Color-Changing Nail Polish
There are several styles of nail polish that change color in sunlight. (One source is Del Sol.)
I went with "clear to rose" nail polish from Educational Innovations.
- Other varieties change from one color to another. I'm bad with colors, so I wasn't confident in my ability to distinguish between, say, "pink" and "cranberry." I stuck with the simplest, which goes from transparent to opaque in the presence of UV light. (Plus, the glittery varieties were a bit sassier than I'd normally go for.)
- I have no idea who manufactures this stuff. The bottle itself has "Solar Magic.com" written on it. (Along with some other writing that's been assiduously scraped off.) The "SolarMagic.com" URL now seems to belong to a Texas Instruments site advertising their solar-energy products. The nail polish is also promoted on a semi-defunct website called Solar Magic Stuff.
Step 2: Find a Clear Top Coat Nail Polish
I wanted to find a clear nail polish that would (hopefully) block some of the incoming UV light.
Ultimately, I chose two:
- "Sally Hansen Ultimate Shield Base & Top Coat." This was the only clear nail polish I could find that promised UV protection on the packaging.
- "Sally Hansen Miracle Gel Top Coat." This one was intriguing because it was the only clear nail polish sold in an opaque bottle with the phrase "NO LIGHT" printed on it in several languages. I figured that with warnings like that, it MUST have interesting properties. (It turns out that it even has its own data sheet. More on this in Step 4.)
I had no idea (and I suspect that most men share my ignorance) what goes on in the nail polish aisle of the department store. It turns out that it's a brightly-lit labyrinth of esoteric vials, tinctures, and varnishes chock full of chemicals that would be kept under lock and key at an auto-parts store.
Step 3: Find a UV Light Meter
It's sensitive to light with a wavelength from 280 nanometers to 400 nanometers, and gives a readout of the UV intensity in milliwatts-per-square-meter, as well as a computed value of the UV Index. (More on the UV Index in the "Notes" section.)
Step 4: (Optional) Visit the Nail Salon
I'm not the kind of man who regularly wears nail polish, and applying consistent coats of nail polish to each of my fingers was a key control in my experiment, so I decided to consult a professional. (Thanks, Ms. Nguyen!)
- Apply a coat of UV color-changing nail polish to each fingernail on both hands. Let it dry.
- Apply a coat of clear "top coat" polish to four fingers on each hand, as in the diagram. Let it dry.
- Apply a second coat of "top coat" to three fingers on each hand. Let THAT dry.
- Apply a third coat of "top coat" to two fingers on each hand. Let it dry.
- Apply a fourth coat of "top coat" to one finger in on each hand.
You should now have an array of fingers with (hopefully) varying sensitivity to UV light. One finger is an "unshielded" control, with no "top coat" applied. The other four fingers each have one, two, three, and four layers of "top coat," respectively.
I learned a lot from my visit to the nail salon. In particular, it was a complete surprise to me that salons come equipped with UV lights to help cure the nail polishes. It turns out that the "NO LIGHT" message on the Sally Hansen Clear Gel Top Coat was to advertise that it can be cured to the first layer of polish without the use of an ultraviolet lamp; the savvy chemists at Coty add a "photoinitiator" chemical that performs the same bonding job using only the UV light in sunlight.
Knowing this, I now wondered how well this product would adhere to a polish by another manufacturer (such as my color-changing nail polish).
Step 5: Test Your UV Metering-Hands!
Now, take your hands out into the sunlight and let it shine on your fingernails. How many will change color?
Unfortunately, we've had a run of cloudy days, so I haven't had a chance to expose my fingernails to intense sunlight.
However, even on a cloudy day, my "control pinkie" (with 0 layers of top coat) turned pink when I stepped outdoors.
First Outdoor Data Point:
Number Of Color-Changing Fingers: 1
Meter's UV Index: 1
Meter's UV Intensity (milliwatts per square meter): 11
Second Outdoor Data Point:
Number Of Color-Changing Fingers: 1
Meter's UV Index: 2
Meter's UV Intensity (milliwatts per square meter): 43
Luckily, I was able to collect another data point without leaving the nail salon! (See next step.)
Step 6: (Optional) UV Lamp Calibration
Unbeknownst to me, nail salons have low-energy UV lamps for curing polishes on fingernails.
(The same fixtures also apply UV light to toenails, apparently. I learned this when I noticed that my shoes were fluorescing while I used the lamp.)
These lamps typically have wavelengths in the UV-A range; 370 nanometers seems to be the most common, according to my research. (See the "Notes" section for more on the UV-A classification.)
Thankfully, my manicurist was also a science buff, and suggested that I test out the lamp. Sure enough, three of my fingernails changed color right away!
Indoor UV Lamp Data Point:
Number Of Color-Changing Fingers: 3
Meter's UV Index: 3
Meter's UV Intensity (milliwatts per square meter): 66
From now on, I'll continue to collect data points in varying amounts of sunlight to see whether or not I can correlate the number of fingernails that change color to the intensity of UV light.
The color change was MUCH more obvious to our eyes than it is in the above photograph (my manicurist gave a "Eureka!" shout when she saw the change). Based on this photo, I'm considering including a color checker card in my future photos of my fingernails.
Step 7: Ruling Out the "Clear Gel" Top Coat
A Partial Result:
In a pre-salon test, the Sally Hansen Miracle Gel Top Coat didn't adhere very well to the underlying nail polish. Either due to a lack of adhesion, or to a lack of UV-blocking properties, it had no observable effect on the UV color-changing nail polish. Even nails with multiple coats of the Miracle Gel applied to them still changed color in the presence of UV light (however, these "layers" may have dribbled away during application).
This product may depend on a reaction between the specific chemicals in Sally Hansen Miracle Gel polishes and the top coat. I can't rule out that this top coat has UV-blocking properties; however, it doesn't adhere well enough to the UV color-changing nail polish to obtain useful measurements.
Step 8: Notes:
Notes On UV Light:
Ultraviolet (UV) light is light that's more energetic than the light we humans can see. Definitions vary, but most scientists consider light with a wavelength between 100 and 400 nanometers (nm) to be "ultraviolet."
(For comparison, the bluish/violet light of certain laser pointers has a wavelength of about 405 nm; right at the edge of the visible portion of the spectrum.)
Scientists studying how UV light affects human health have found it handy to classify UV light in one of three categories:
- UV-A (315 - 400 nm)
- UV-B (280 - 315 nm)
- UV-C (100 - 280 nm)
In particular, they realized that UV light in the 280-315 nm range was responsible for the majority of sunburn (or "erythema"). They called this relatively narrow range "UV-B" light, and paid careful attention to it.
By contrast, the low-energy UV light called "UV-A" (such as the lamp I used at the nail salon) had much less of an effect on human heath. The higher-energy UV light in the UV-C range was found to be much more harmful to human health.
However, not all wavelengths are equally good at passing through Earth's atmosphere. In fact, nearly all of the sunlight that falls in the UV-C range is blocked by the Earth's atmosphere. UV-A light, by comparison, does a much better job of sailing straight through the atmosphere and reaching the Earth's surface.
For a while, scientists were scratching their heads about how to balance these concerns: UV-B light punches above its weight when it comes to causing sunburn, but it's not as good at reaching humans as UV-A light is; UV-C light is devastating, but doesn't turn up in the sunlight that reaches Earthlings. What to do?
In the 1990's, a group of Canadian scientists came up with a clever way of condensing all of these considerations into a single, easy-to-digest number: the UV Index.
They combined measurements of how well light penetrates the atmosphere (where UV-A does the best) versus its sunburn-causing potential (UV-B's specialty) into a single number that's normally between 0 and 10. (Unless you're an astronaut or a welder, you probably won't encounter a UV index above 11. At least, I hope you won't...)
What Can Our Fingers Measure?
Technically, our fingernails should only be able to measure the intensity of UV light in a certain range of wavelengths. They don't take into account the atmospheric and human-health factors that are used to compute the UV Index. (They're just fingernails, after all!)
However, by collecting more data points, I hope to be able to correlate the intensity of UV light with the number of coated fingernails that change color. Now, all I need is a break in this cloudy weather!
Sharing Your Results:
Have you tried this experiment? Share your results in the comments section! I'd be particularly interested to learn about your results with other varieties of nail polish and top coats!
If you don't have access to a portable UV meter, you can get a daily prediction of the UV Index for your location from the National Weather Service. Or, find an Internet-connected weather station near you that reports the UV intensity, and use those figures for your measurements. Good luck!
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