Why Does Packing Tape Glow When You Rip It Off in a Dark Room?
― At the Moment of Peeling, Tiny Sparks Jump Across the Gap
In a pitch-dark room, yank tape off the roll and a faint blue-white light flashes along the peel line. The glow doesn't come from the glue or the tape itself. It comes from tiny sparks: electric charge, created as the tape is pulled apart, leaps across the air in the gap.
It's the night of a move. You've turned off the light and are sealing a cardboard box.
You pull the packing tape off the roll in one go. For an instant, the peel line seems to glow bluish. The ripping sound seems louder than usual, too.
Is it a trick of the eye? No, the light is real. Once your eyes have adjusted to the dark, anyone can see it.
Only Two Reasons It Glows
Where tape stuck to tape, a few charged particles have shifted from one surface to the other. When you peel, positive and negative are pulled apart, and a high voltage builds up in the gap.
Once the voltage passes a few hundred volts, the air in the gap starts to conduct electricity. The nitrogen in the air then gives off light. That is the blue-white glow.
So the glow from tape is much like the "zap" from a doorknob in winter. The difference is that many very small sparks fire one after another along the peel line.
What Happens in the Gap Between the Tapes?
When two different materials touch, a few charged particles (electrons) move from one to the other at the contact surface. No rubbing is needed. Close contact is enough. With tape, the pair is the sticky side and the back of the tape underneath it.
While the tapes are stuck together, positive and negative sit right next to each other, so there is almost no voltage. But once you peel and a gap opens, things change. The amount of charge stays the same while the distance grows, so the voltage rises in proportion to the width of the gap.
Try the slider in Figure 1 to change the gap width. A gap about the thickness of one hair (roughly 0.1 mm) is enough to push the voltage to nearly 300 volts, by this calculation.
Why Blue-White?
Inside a spark, electrons are pulled along fast by the electric force and crash into air molecules. Each molecule that is hit gives up the energy it received as light.
Nearly 80% of air is nitrogen. Much of the light nitrogen emits is invisible ultraviolet, and the part we can see leans toward blue-violet. That is why the tape's glow, lightning, and static sparks all look a similar blue-white.
The light flickers in fine bursts along the peel line. That's because tape doesn't peel smoothly at a steady speed. It keeps catching, then letting go all at once. The ripping sound comes from this same jerky peeling.
In 2008, a US research team peeled tape inside a chamber with the air pumped out. They found it gave off extremely brief flashes of X-rays. They reported that the X-rays were even strong enough to take a photo of the bones in a finger. Without air, the electrons keep accelerating without hitting anything, and strike the tape at high speed. In the air of an ordinary room, electrons collide with air molecules and stop almost at once, so there is nothing to worry about.
Rub two lumps of rock sugar or sugar cubes together in the dark, or snap them, and they glow faintly. This is thought to happen because, as the crystal breaks, charge is pulled apart between the broken faces. Light made by peeling, rubbing or breaking is known as triboluminescence.
Summary
When tape sticks, a little charge moves across the touching surfaces. Peel it and a gap opens: the amount of charge stays the same while the distance grows, so the voltage climbs past a few hundred volts. The air then conducts, tiny sparks jump, and nitrogen glows blue-white.
The glow of tape is a palm-sized
"little lightning bolt" made by the hand that peels it.
The same mechanism is behind the winter doorknob "zap," covered in Why Do You Get a Static Shock in Winter?, and the story of invisible static setting off a fire is in Why Touch Metal Before Refuelling?.
- Make the room completely dark and wait about 10 minutes for your eyes to adjust.
- Pull packing tape (cloth or clear) off the roll in one go and watch the peel line closely. Compare the glow with what you see when you peel slowly.
- Rub two lumps of rock sugar hard together in the dark. Compare a dry day with a rainy one.
On humid days, the charge on the tape's surface leaks away through moisture, so it glows less. Don't peel tape close to your face, and ball up the used tape before throwing it away.
For the Curious ― Terms, Formulas, and Links to TextbooksEach part is marked with its level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school physics
- High school+Advanced high school material, or textbook sidebar content
- UniversityUniversity-level courses not taught in high school (electromagnetism, discharge engineering, solid-state physics)
- ResearchNot yet settled even at university level, and still being studied by researchers
Middle schoolTerms: This Phenomenon Has a Name
- Triboluminescence: light given off by a material when a force is applied, such as peeling, rubbing or breaking.
- Static electricity (charging): positive or negative charge building up unevenly on a surface. It can happen from touching alone, which is called contact charging.
- Discharge: stored charge rushing through air or another medium all at once. It comes with sparks and light.
Middle schoolHigh schoolCheck It with a Formula: What Voltage Does a 1 mm Gap Give?
Treat the two surfaces of the peeled tape as two flat plates carrying positive and negative charge. For the charge level, we assume an example value taken to be about the upper limit a surface can hold in air.
| In symbols | V ＝ σ × d ÷ ε0 |
| In words | Gap voltage ＝ charge per square metre on the surface × gap width ÷ permittivity of free space |
| Where it comes from | Between two flat plates, Gauss's law gives an electric field of σ ÷ ε0. Voltage is "field × distance," so if the charge stays the same, the voltage is proportional to the gap width (the same as the capacitor formula). |
| Symbol | Meaning and unit |
| V | Gap voltage (volts) |
| σ | Charge per square metre of surface (coulombs per square metre) |
| d | Gap width (metres) |
| ε0 | Permittivity of free space (farads per metre). Nearly the same value in air |
| Charge σ (assumed) | About 26 microcoulombs per square metre (taken to be about the upper limit a surface can hold in air) |
| Permittivity of free space ε0 | 8.85 picofarads per metre |
| Gap width d | 1 millimetre |
| Lowest voltage at which a spark can jump in air | About 300 volts (said to apply when the gap is a few micrometres) |
| Voltage of one AA cell | 1.5 volts |
| Field σ ÷ ε0 (unit: million volts per metre ＝ kilovolts per millimetre) | 26 ÷ 8.85 ≒ 2.94 |
| Voltage for a 1 mm gap (volts) | 2.94 × 1000 ＝ 2940 |
| Voltage for a 0.1 mm gap (about one hair) (volts) | 2940 ÷ 10 ＝ 294 |
| The 1 mm case as a number of AA cells | 2940 ÷ 1.5 ＝ 1960 |
A gap just one hair wide brings the voltage up to about 300 volts, the lowest line for a spark. Open it to 1 mm and it equals roughly 2000 AA cells. Even when the charge itself is tiny, the distance you pull it apart pushes the voltage up.
High schoolHigh school+Why Does the Voltage Rise When the Charge Stays the Same?
High schoolTape being peeled acts like a capacitor whose plates are moving apart. When the charge Q is constant, the capacitance falls in inverse proportion to the spacing, so the voltage V ＝ Q ÷ C rises in proportion to the spacing. The work done by the peeling hand is stored in the gap as electrostatic energy.
High school+The voltage at which air starts to conduct does not simply keep rising with the gap. If the gap is too narrow, electrons don't hit enough molecules on the way, and discharge becomes harder instead. For this reason the spark voltage has a minimum, which in air is about 300 volts.
UniversityPaschen's Law and Nitrogen Emission
The fact that the spark voltage (breakdown voltage) depends only on the product of gas pressure and gap width is called Paschen's law. The colour of the tape's glow comes mainly from nitrogen molecules' "second positive system" of emission, whose strong lines are said to lie near 337 nanometres in the ultraviolet. The X-rays in a vacuum are bremsstrahlung, produced when accelerated electrons strike a material and are deflected. How the charge moves in the first place is treated as contact (triboelectric) charging in solid-state physics and surface chemistry.
📖 Derivations and further reading: Paschen's law (Wikipedia, in Japanese) ／ Triboluminescence (Wikipedia, in Japanese)
ResearchWhat Is Still Not Well Understood
- What actually moves on contact? Whether contact charging moves electrons, ions or tiny bits of material differs by material, and the debate continues.
- Why does tape hold so much charge? In vacuum experiments, the charge built up locally is estimated to be far greater than the rough upper limit in air.
- Can discharge alone explain the glow from crystals? For sugar and similar materials, some researchers think that besides the discharge light, the crystal itself also gives off light.
In other words, this article too is "an explanation of what we know so far." Even a single roll of tape holds questions that remain unanswered.
Links to Textbooks (by Level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science, Year 2: "Static Electricity and Current" | Touching builds up charge; discharge makes light |
| High school | Physics: "Electric Field and Potential," "Capacitors" | The formula where voltage rises with the gap |
| High school+ | Physics (advanced): "Gas Discharge" | The minimum spark voltage |
| University | Electromagnetism, discharge engineering, spectroscopy | Paschen's law, nitrogen emission, bremsstrahlung |
| Research | Surface science, tribology | How contact charging works; how peeling relates to light emission |
| ― | Links to daily life | Winter static, humidity and charging, discharging before refuelling |
- Camara, C. G., Escobar, J. V., Hird, J. R., Putterman, S. J. (2008) Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape. Nature 455, 1089–1092.
- 摩擦発光 (Triboluminescence) ― Wikipedia, in Japanese
- パッシェンの法則 (Paschen's law) ― Wikipedia, in Japanese
- Sweeting, L. M. (2001) Triboluminescence with and without air. Chemistry of Materials 13, 854–870.
- High school physics textbook units on "Electric Field and Potential" and "Capacitors"
※This article is a general-audience science explainer. The figures given are rough guides to help you grasp the mechanism. How much charge builds up varies greatly with conditions, and the values here rest on an assumed example.