UV Beads Sunscreen Experiment: Proof You Can See

A child who has been told to put sunscreen on has no evidence that it does anything. It is not visibly doing a job. This UV beads sunscreen experiment closes that gap in about thirty seconds: a handful of plastic beads that are off-white indoors burst into purple, pink and orange the moment they reach direct sunlight, and the ones you have smeared with sunscreen stay stubbornly pale beside them. Both sets are sitting in the same light. The only difference is a film of lotion blocking something nobody in the family can see. That is the whole argument for sun protection, demonstrated rather than asserted, and it takes one minute outdoors.

Key Takeaways

• Photochromic UV beads change colour in sunlight within 10 to 30 seconds and fade back in shade within a minute or two, so you can run the comparison repeatedly.
• Sunscreen smeared on a clear bag of beads keeps them pale while uncovered beads beside them go bright. The contrast is unmistakable in photographs.
• The beads also colour up in open shade and under light cloud, which is the most useful public-health lesson in the whole activity.
• The beads cannot compare SPF numbers. They respond mostly to UVA, while SPF is defined by UVB-driven sunburn. Claiming the beads prove SPF 50 beats SPF 30 is not supportable.
• Best for ages 6 to 13. The beads are a choking hazard for children under three.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Photochromic UV beads	30–50	Science suppliers, craft shops, often sold as “UV detecting beads”	There is no household substitute. Sun-print paper behaves similarly but is single-use
Small clear zip bags	5	Supermarket, the smallest size	Clear food wrap stretched over a jar lid, taped tight
Broad-spectrum sunscreen SPF 30+	A pea-sized blob per bag	Pharmacy	n/a
Second sunscreen, SPF 15	A pea-sized blob	Pharmacy	Skip this bag; the comparison is optional and limited anyway
Plain moisturiser with no SPF	A pea-sized blob	n/a	This is the control that matters most. It looks like sunscreen and does nothing
White card	1 sheet	n/a	A white plate or a sheet of printer paper
Phone camera	1	n/a	Photograph every set in one frame, in the same light
Hat and sunscreen for the child	n/a	n/a	Not optional. You are going outside into real UV
Scrap cotton and nylon fabric	A square of each	An old t-shirt, a nylon jacket lining	For the fabric test

Time: prep 10 min, experiment 20 min, cleanup 5 min Cost: a bag of 250 UV beads costs about the same as a takeaway coffee and lasts years Mess level: low, but sunscreen on a camera lens is a nuisance; wipe your hands

Do the Experiment: The UV Beads Sunscreen Experiment

Step 1 - Record the indoor baseline

Indoors, away from windows, look at the beads. They should be white, cream or very pale. Put 6 to 8 beads into each of five clear zip bags and photograph them side by side on the white card. This photograph is your zero reading, and without it the later comparison is just an impression.

Step 2 - Treat four of the five bags

Label the bags: A nothing, B SPF 15, C SPF 30 or higher, D plain moisturiser with no SPF, E nothing but destined for the shade. Smear a pea-sized blob of the relevant product on the outside of each bag and spread it to an even, thin, complete film. Coating the bag rather than the beads is cleaner, more even, and reusable.

Step 3 - Ask for a prediction

Ask which bags will stay pale and which will change. Specifically ask about bag D, the plain moisturiser. Most children, and quite a few adults, expect any white cream to help. Write the predictions down before you go outside.

Step 4 - Take all five outside together

Put bags A to D on the white card in direct sunlight, and bag E in full shade under a tree or an umbrella at the same moment. Start a timer. Photograph at 15 seconds, 30 seconds and 60 seconds from the same position.

What you should see: Bag A goes vivid within 10 to 30 seconds, usually purple, magenta or orange depending on the bead. Bags B and C stay close to white, with a faint blush at the edges where the film is thin. Bag D changes almost as much as bag A, because moisturiser without a sunscreen filter does essentially nothing to ultraviolet. Bag E, in the shade, changes too, just more slowly and less intensely.

Step 5 - Run the window test

Bring one untreated bag back inside and place it on a windowsill in bright sun, behind the glass.

What you should see: The beads still colour up, often taking two or three times as long as outdoors. Ordinary window glass absorbs most UVB but transmits a substantial share of UVA, so a sunny room is not a UV-free room. This is a genuinely surprising result for most families.

Step 6 - Test fabric, dry and wet

Lay a square of white cotton over one bag and a square of nylon over another, then take both into the sun. Repeat with the cotton square soaked in water.

What you should see: Dry cotton blocks a lot but not everything. Wet cotton blocks noticeably less, because water fills the gaps between fibres and reduces scattering. This is why a wet white t-shirt is poor sun protection, which is worth knowing before a day at the pool.

Step 7 - Watch them reset

Bring everything indoors and watch.

What you should see: The colour drains away over roughly one to three minutes, faster in a warm room than a cold one. The beads are reusable hundreds of times. The temperature dependence is a clue to the mechanism.

What’s Actually Going On

Sunlight contains ultraviolet radiation, which is light with wavelengths shorter than violet, from about 400 nm down. The NASA explainer on ultraviolet waves sets out the standard divisions: UVA from roughly 315 to 400 nm, UVB from 280 to 315 nm, and UVC below that, which the atmosphere absorbs almost completely. Human photoreceptors do not respond to any of it, so UV is genuinely invisible rather than merely hard to see.

The beads contain a photochromic dye, usually a spirooxazine or a naphthopyran, dispersed in a plastic matrix. In the dark, the molecule sits in a closed, colourless form. Absorbing a UV photon breaks a single carbon-oxygen bond in the spiro ring, and the molecule opens up into a flat, extended, conjugated structure called the merocyanine form, which absorbs visible light strongly and therefore looks coloured. The reaction reverses thermally, which is why the beads fade faster in a warm room, a relationship worked through in detail by Maurel and colleagues in Molecular Crystals and Liquid Crystals in 2005. What you see outdoors is a photostationary state: an equilibrium between UV-driven opening and thermal closing, which is the point Jerry Bell made in the Journal of Chemical Education in 2001.

Sunscreen works by two mechanisms at once. Organic filters such as avobenzone and octocrylene absorb UV photons and dissipate the energy as heat. Inorganic filters, zinc oxide and titanium dioxide, mostly absorb as well, with some scattering and reflection; the common claim that they work purely by reflecting light is an oversimplification. Either way, photons that get absorbed in the sunscreen film never reach the bead, and the dye never opens.

Now the honest limitation, which is the part most activity write-ups skip. These beads respond primarily to UVA, with peak sensitivity somewhere around 350 to 370 nm. The SPF number on a bottle is defined by how much longer protected skin takes to redden, and reddening is driven mainly by UVB. As the US Food and Drug Administration explains on its sunscreen page, SPF measures protection against sunburn, and separate “broad spectrum” testing covers UVA. So the beads are testing something the SPF number does not describe. Your bag B and bag C will often look nearly identical, and that is correct behaviour, not a failed experiment. The claim the beads genuinely support is the big one: sunscreen blocks invisible light, and moisturiser does not.

The shade result deserves its own sentence. Roughly half of the UVA reaching you on a clear day arrives as scattered skylight rather than direct sun, which is why bag E changes colour under a tree and why people burn on overcast days. The EPA’s UV Index scale is built around total UV exposure for that reason.

How to Explain It to Your Kid

Ages 5–8: There is a colour the sun has that your eyes can’t find

Tell them the sun sends out a kind of light nobody can see, and the beads are a detector for it, like a smoke alarm for sunshine. Let them run the bags in and out of the shade themselves, which is the part they will want to repeat ten times. Then let them hold the sunscreen bag and the plain-cream bag side by side in the sun and say which one is “wearing armour.” No chemistry required.

Ages 9–12: UVA, UVB and why shade is not zero

Introduce ultraviolet and the UVA/UVB split, and give the wavelength numbers, since they are concrete. Then focus on the shade test, because it carries the real-world consequence. Have them time how long bag E in the shade takes to reach the same colour bag A reached in direct sun, and work out the ratio. That number, typically two to five times longer, is more persuasive than any warning, and it connects with why the sky is blue, since the same scattering sends UV down from the whole sky.

Ages 13+: Photostationary states and the limits of the measurement

Give them the mechanism: reversible ring-opening to the merocyanine form, thermal reversion, and the resulting photostationary state whose position depends on both UV intensity and temperature. Then push on the experimental design. Why can this experiment not rank SPF values? What would you have to measure instead, and with what instrument? How would you control for film thickness, which is the dominant uncontrolled variable here? A teenager who can articulate why their own result does not support a popular claim has learned something better than the claim.

The question to ask: “If we put a bag of beads at the bottom of a bucket of water, 20 cm down, would they change colour? Decide before we try it, and say what that would mean for swimming.”

Why Teaching This Early Pays Off

Skin cancer risk accumulates over a lifetime and childhood exposure matters, which makes sun protection one of the few health habits where a demonstration at age eight has a plausible long-term payoff. The honest framing is narrow, though: this makes a child likelier to accept sunscreen without argument, and no evidence says one afternoon changes behaviour years later.

The deeper lesson is about instruments. Ultraviolet is the first thing many children meet that is physically real, biologically consequential and completely invisible to human senses. Accepting that the beads are telling the truth about something the eye cannot verify is a small act of scientific trust, and it is the same act required later for X-rays, radio waves, carbon monoxide detectors and blood tests. The National Research Council’s science framework lists using instruments to extend the senses among the practices children should build, and this is an unusually clean example.

It is also a good place to practise reading a label sceptically. SPF is a legally defined quantity that measures one thing, and plenty of marketing invites you to believe it measures more. A child discovering that their own experiment cannot distinguish SPF 15 from SPF 30 beats any lecture about critical thinking.

Before You Start: What Parents Should Know

Best age	6 to 13. Under 6 is fine with an adult handling the beads
Supervision	Constant for under-6s, because of the beads. Adult decides exposure time outdoors
Real hazards	The beads are small and are a choking hazard for children under three; count them out and count them back. You are deliberately going into UV, so put a hat and sunscreen on the child, keep the session under 20 minutes, and avoid 11am to 3pm where you can. Do not look at the sun. Do not put sunscreen on a baby under six months; the AAP advises shade and clothing instead
Don’t	Don’t substitute a UV torch for sunlight if you want the sunscreen comparison to mean anything, and never shine a UV torch toward anyone’s eyes. Don’t let a child apply sunscreen to the beads and then handle food

When It Doesn’t Work

• The beads are already coloured indoors. They are picking up UV from a nearby window or a fluorescent tube. Move to an interior room, let them fade for five minutes, and re-photograph the baseline.
• The sunscreen bag changes almost as much as the control. Your film is too thin or patchy. Sunscreen performance depends heavily on how much you apply, which is also true on skin. Use a clearly visible layer and spread it right to the edges.
• Nothing changes outdoors at all. Check you are in direct sun rather than behind glass or deep shade, and check the beads are photochromic UV beads rather than ordinary coloured craft beads. In midwinter at high latitudes the UV index can be near 1, which slows everything down.
• The beads stop working after a season. Photochromic dyes do fatigue with repeated cycling, and heat accelerates it. Store them in a dark, cool, closed container, not on a sunny windowsill.
• SPF 15 and SPF 50 look identical. That is the expected result and the reason this article exists. The beads respond to UVA, SPF describes UVB. Report it as a finding, not a failure.

Take It Further

• Build a UV index diary. Photograph one untreated bag at the same time and place daily for two weeks, next to a printed colour chart, and record the official UV index from a weather service. Plotting bead colour rank against the published index gives you a citizen-science data set with an external reference.
• Test the water depth question. Put a bag of beads at 2, 10 and 20 cm under water in a clear container in full sun, timing the colour change. UV penetrates water well, which is why people burn while swimming, and the measured slowdown with depth is modest and surprising.
• Compare clothing properly. Rank five fabrics by how long they delay the colour change: white cotton, dark cotton, nylon, a loose knit and a dedicated sun shirt, each dry and then wet. Record the times. This produces a ranking you will actually use when packing for a beach trip, and it dovetails with sun prints, which use the same ultraviolet to make a photograph.

What to Watch For Over the Next 3 Months

• Week 4: They apply sunscreen without an argument, or at least argue less. Watch for them mentioning shade as partial rather than total protection.
• Month 2: The word “ultraviolet” is used correctly, separately from “heat” and “brightness”. The three get conflated constantly, and separating them is real progress.
• Month 3: If they ask why the sun feels hot through a window but the beads react slowly there, they have noticed that infrared and ultraviolet behave differently in glass. That is the right question and it opens up the whole spectrum.

Frequently Asked Questions

Can this experiment tell me whether my sunscreen is any good?

It can tell you that your sunscreen blocks UVA, which plain moisturiser does not. It cannot rank SPF values, because SPF is defined by UVB-driven sunburn and these beads respond mainly to UVA. Read the label for “broad spectrum” and SPF 30 or higher.

How long do UV beads last?

Hundreds of cycles, often several years of occasional use. Photochromic dyes do fatigue, and heat and prolonged UV accelerate it. A dark, cool container between sessions makes the difference between a season and a decade.

Is it safe to do this at midday in summer?

It is safe for the beads and less ideal for the child. The beads only need 30 to 60 seconds, so there is no reason to stand in peak sun. Do it in the morning or late afternoon, with a hat, and bring everything back indoors as soon as you have your photographs.

Why do the beads change colour behind a window?

Because ordinary window glass absorbs most UVB but lets through a significant fraction of UVA. A bright room by a window is not UV-free.

Can I use a UV torch instead of going outside?

For showing that UV exists, yes. For the sunscreen comparison it is misleading, because most cheap UV torches emit a narrow band near 395 nm, at the very edge of the spectrum. Never point one at anyone’s eyes.

---

About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

---

Sources

1. US Food and Drug Administration. “Sunscreen: How to Help Protect Your Skin from the Sun.” https://www.fda.gov/drugs/understanding-over-counter-medicines/sunscreen-how-help-protect-your-skin-sun
2. Trupp, T. (2001). “Putting UV-Sensitive Beads to the Test.” Journal of Chemical Education, 78(5), 648A. https://doi.org/10.1021/ed078p648a
3. Bell, J. A. (2001). “Visualizing the Photochemical Steady State with UV-Sensitive Beads.” Journal of Chemical Education, 78(12), 1594. https://doi.org/10.1021/ed078p1594.3
4. Maurel, F., Aubard, J., Samat, A., Rajzmann, M., & Guglielmetti, R. (2005). “Interplay Between Theory and Experiment in Organic Photochromism: Example of Spirooxazine Ring Opening/Closing Equilibrium.” Molecular Crystals and Liquid Crystals, 430, 221–226. https://doi.org/10.1080/15421400590946424
5. NASA Science. “Ultraviolet Waves.” Tour of the Electromagnetic Spectrum. https://science.nasa.gov/ems/10_ultravioletwaves/
6. US Environmental Protection Agency. “UV Index Scale.” https://www.epa.gov/sunsafety/uv-index-scale-0
7. World Health Organization. “Radiation: Ultraviolet (UV) radiation.” https://www.who.int/news-room/questions-and-answers/item/radiation-ultraviolet-(uv)
8. American Academy of Pediatrics, HealthyChildren.org. “Summer Sun, Heat & Air Quality: Tips to Keep Kids Safe.” https://www.healthychildren.org/English/safety-prevention/at-play/Pages/Sun-Safety-and-Protection-Tips.aspx
9. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165