# What are these alternating bright and dark rings in fluorescent lamp?

I'm quite surprised by these regularly spaced rings of alternating brightness in a fluorescent tube. These are also moving along the tube and only appear when the voltage is low. What are these and does the pattern have anything to do with AC frequency?

• I think that this rather is “quantum mechanics” than “plasma physics”, the lamp should not get this hot to produce actual plasma. – Martin Ueding Aug 6 '17 at 11:04
• Fluorescent lights work exactly because the gas inside is ionized into a plasma. Hg vapor is ionized, which produces UV photons when the electrons re-combine with the + ions, and these UV photons strike the phosphor coating on the inside of the glass, and the atoms in this phosphor coating undergo fluorescence, absorbing UV photons and re-emitting visible light photons. – Apollonius Aug 6 '17 at 11:46
• Closely related: physics.stackexchange.com/questions/266010/… – dmckee --- ex-moderator kitten Aug 6 '17 at 18:08
• Is this visible with the naked eye, or only on camera? I wonder if it could be a rolling shutter effect? – Wossname Aug 6 '17 at 21:37
• I swear this is not a camera trick as I can see with my own eyes – Knight Aug 7 '17 at 3:33

This reminds me of the Franck-Hertz experiment where similar patterns occur.

At low voltage, the free electrons in the tube will be accelerated by the voltage until they have enough energy to excite a gas atom by hitting it. The atom eventually falls back to the ground state and emits the light. Due to quantization of the excitation levels, this appears in spacing.

When the voltage is high enough, most of the gas is excited such that the electrons can move freely and therefore the pattern is gone.

• Interesting , thank you for heading me towards something to read. – Knight Aug 6 '17 at 12:47
• The Franck-Hertz experiment is done with DC current, not AC current, but I'll allow that it could just switch polarity at 60Hz (USA). However, if this is truly the same effect as the Franck-Hertz experiment, then each gap should represent 10.437 volts of potential difference for singly ionized mercury vapor; however, we should also expect that the center of the bulb would be noticeably dimmer than the ends. – Apollonius Aug 8 '17 at 7:02

It's been suggested that the pattern is due to a standing density waves (sound) in the plasma inside the tube, where the orange/blue spots might correspond to nodes/antinodes in the standing wave. You can attempt to find the frequency of some electrical signal that might be causing this by first measuring the distance between a pair of blueish (or a pair of pinkish) spots, and call that the wavelength $\lambda$. Then compute a rough theoretical estimate of the speed of sound in a mercury vapor plasma from:

$v = 9800\times \sqrt{ \dfrac{\gamma T_e}{\mu} }$ (meter/sec)

where $\gamma = 1$ for reasons I won't go into, $T_e$ is the electron temperature and $\mu=200.6$ which is the mass ratio of mercury (Hg) to a proton. We can estimate an electron temperature from the 1st ionization potential for mercury, which is about 10.4 volts. Free electrons will need to be accelerated to a kinetic energy (KE) of 10.4 eV in order to ionize mercury atoms. The KE of electrons in a plasma is related to their electron temperature by $\langle KE\rangle=kT_e$ where $k$ is Boltzmann's constant. So we get an electron temperature of roughly

$T_e = (10.4~eV)(1.602\times 10^{-19} J/eV)~~/~~(1.38\times 10^{-23} J/K) = 1.21\times 10^5 K$

This seems about 10 times higher than the actual thermal temperature of the plasma inside the tube, but there are reasons why this is so (Google "electron temperature")

So... $v = 9800 \sqrt{\dfrac{1.21 \times 10^5}{200.6}}$ = $2.41\times 10^5$ m/s. (Yeah, that's fast)

Now, using $f=v/\lambda$, you can calculate the frequency of some signal causing the pattern.

$\it{Example}$ : if $\lambda$ = 20 cm, then $f=$1.20 MHz, which is definitely a frequency associated with AM radio carrier waves.

Maybe this analysis is correct, and maybe there's some other mechanism making the pattern, but go ahead and measure $\lambda$ and turn the crank to see what you get for frequency $f$. Then try building an RF transmitter you can vary about the frequency value you calculated, and see if you can make the pattern change as you change the frequency of the transmitter near the bulb.

Good luck!

These rings are called "Faraday cones", according to Wikipedia. They occur when the mercury vapor pressure is low. I have most often seen them when a fluorescent bulb is colder than 70 degrees F, especially in bulbs nearing the end of their life. They may disappear and the brightness increase as the bulb warms up. Perhaps this can lead you to a mechanistic explanation.