A light bulb using alternating current at 60Hz turns on and off 120 times per second but in a monitor 60Hz means 60 refreshes per second.

Why is that? Shouldn't 60Hz mean 120 refreshes?

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    \$\begingroup\$ long story short: no, it shouldn't. The lamp flashing has nothing to do with a "refresh", a monitor is not an incandescent light bulb or comparable to one. \$\endgroup\$ Sep 28, 2020 at 19:37
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    \$\begingroup\$ A light bulb is a frequency doubler... because it displays P=U^2/R (yes, I know it is not really true, looking at the lightbulb later filters out higher harmonics in the perception) and square of sin t is sin 2t (plus dc offset) \$\endgroup\$
    – lalala
    Sep 30, 2020 at 12:45
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    \$\begingroup\$ I think your misunderstanding stems from your opening statement being wrong:- "A light bulb using alternating current at 60Hz turns on and off 120 times per second". In reality a light bulb using alternating current at 60Hz turns on and off 60 times per second. It turns on 60 times and it turns off 60 times a second. \$\endgroup\$
    – TopCat
    Sep 30, 2020 at 20:03
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    \$\begingroup\$ @TopCat, in one AC cycle, the voltage goes from zero to positive, back to zero, then negative, and again back to zero. That is, in 60 Hz AC, there's one positive half-cycle and one negative half-cycle sixty times a second. But since the lightbulb is pretty much just a resistor, and doesn't really care which way the current flow, it lights up on both positive and negative half-cycles, bringing the total to 120 times per second. (That is, assuming it cools off enough during the brief zero-crossings that it can be said to turn "off" between the half-cycles.) \$\endgroup\$
    – ilkkachu
    Sep 30, 2020 at 21:12
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    \$\begingroup\$ That is, assuming it cools off enough during the brief zero-crossings that it can be said to turn "off" between the half-cycles. -- It doesn't. As you have already suspected, the filament in an incandescent bulb continues glowing long enough to produce a continuous light between successive electrical cycles. \$\endgroup\$ Sep 30, 2020 at 23:46

6 Answers 6


Hertz means 'cycles per second', where a cycle is an event that's repeated.

With the light bulb, that's cycles of the mains supply frequency. Being a balanced a.c. waveform, that crosses 0 V twice per cycle. That causes the 100 light bulb flashes per second from the 50 Hz supply, or 120 flashes from a 60 Hz supply.

With the monitor, a cycle is a top-to-bottom refresh of the display pixels on the phosphor or LCD screen. There's no a.c. aspect in play here. If anything, the term could be applied to the frequency of a monitor's VSYNC signal, if its got one, but that's not why the term arose.

The unit 'Hz', originally used for a.c. waveforms, was the one later used for refresh rates in television and thence in monitors. The use of Hz was to do with how early television sets derived timing from the mains frequency.

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    \$\begingroup\$ They key difference is that a TV lights up only on the "down" portion of the beam's vertical travel, while the bulb lights at both the "top" and "bottom" of the waveform. \$\endgroup\$ Sep 30, 2020 at 0:46
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    \$\begingroup\$ @2012rcampion, that's not a key difference, though - the light bulb and monitor are completely different, happening to use one engineering term in different ways. \$\endgroup\$
    – TonyM
    Sep 30, 2020 at 13:16
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    \$\begingroup\$ Hertz actually means “things per second”, and in waveforms, a wave cycle is one of the “things” that are counted, not half-waves. Centuries ago, they spoke of “Doppelschwingungen pro Sekunde” (double waves/vibrations/oscillations per second) not “Schwingungen pro Sekunde” (waves per second) when referring to e.g. sound frequencies (pitches) to make this clear. \$\endgroup\$
    – mirabilos
    Oct 1, 2020 at 15:01
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    \$\begingroup\$ @mirabilos, the SI (and NIST) definition of Hertz (Hz) is cycles per second. \$\endgroup\$
    – TonyM
    Oct 1, 2020 at 15:25
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    \$\begingroup\$ @TonyM wrong (I just checked the SI brochure). SI defines Hertz as “frequency” and Hz = s⁻¹ and specifically has to say this about cycles: Note that in some countries, frequency values are conventionally expressed using “cycle/s” or “cps” instead of the SI unit Hz, although “cycle” and “cps” are not units in the SI. It says “The hertz shall only be used for periodic phenomena”, so it’s a unit for periodically occurring things (can be anything as long as it occurs periodically). \$\endgroup\$
    – mirabilos
    Oct 5, 2020 at 0:30

Light bulbs blink at twice the mains frequency because the AC waveform goes to negative side, and has two pulses per cycle:

AC mains waveform

Compare that to the brightness you would see with an old time CRT monitor, or nowadays with VR headsets:

Display backlight brightness

The pulses = frequency x 2 behavior only applies to cases where the waveform goes to negative side and both half-cycles cause separate pulses.

  • 1
    \$\begingroup\$ Picture is worth 1k words \$\endgroup\$
    – Cireo
    Oct 30, 2020 at 18:52
  • \$\begingroup\$ Your second diagram suggests to me that the 'flyback' phase of the electron beam takes just as long as the screen scan. This surprises me. From my memory of how CRT TV works, I would expect the display brightness to be more of a sawtooth shape. Did I understand your diagram correctly? \$\endgroup\$
    – Qsigma
    Oct 31, 2020 at 18:02
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    \$\begingroup\$ @Qsigma It will depend on what size of an area you are looking at. A small area would have a short bright pulse and a long dark time, while average brightness over a larger area would be more even 50-50 waveform. And phosphor persistence would matter also. But in any case, one pulse per scan. \$\endgroup\$
    – jpa
    Oct 31, 2020 at 19:38

Because you're referring to completely different cycles with different purposes.

AC lighting uses a sinewave, because that's what works best with power distribution equipment. That's really the end of that conversation.

Monitor sweeps use a sawtooth pattern, because it paints the picture from top to bottom, and then snaps back up to the top very quickly to do it again.

In both cases the thing repeats 60 times a second.

Note that TV monitor frequency is not inherently tied to AC power line frequency. 60Hz NTSC TVs could work on 50Hz. and in fact NTSC, PAL and SECAM refresh frequencies don't even match their regional power frequency. In actuality, NTSC was 59.94 FPS. You could see this on a mildly defective TV, as the AC power ripple crawls up the screen slowly. That may have been the point; so the ripple crawls rather than jumps spasmodically.

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    \$\begingroup\$ Not quite coincidence - pre-colour the field rate was locked to the AC power line frequency to minimize the effects of power line interference. Conveniently it also matches requirements for flicker visibility. When colour TV was developed they kept a frequency close to the existing rate so that existing TVs could render the signal as monochrome. \$\endgroup\$ Sep 28, 2020 at 19:54
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    \$\begingroup\$ My understanding is that the 59.94 (originally half that at 29.97) was so that they could slip in some sync information into that extra 0.03 FPS. On a defective old TV, you could actually see that extra info rendered on the screen as a pattern below the image. Not really relevant on modern digital signals of course. \$\endgroup\$ Sep 29, 2020 at 20:46
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    \$\begingroup\$ @KevinWhite: On many older sets, picture geometry will change slightly depending upon where in the beam is when the DC power supply capacitors topped off during the peak of each AC line cycle. Having the picture "breathe" a few times per minute isn't as bad as having it breathe ten times per second (e.g. if broadcast at 50 fields/second) would be. \$\endgroup\$
    – supercat
    Sep 29, 2020 at 22:13
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    \$\begingroup\$ @DarrelHoffman - That's the Closed Caption (or Teletext in other countries) information you can see. It occupies complete lines just after the frame sync. They are intended to be off the top of the screen in the overscan area - it doesn't take alter the timing. I haven't really found a rationale for the choice of the 59.94 frames per second although the timing can be derived from an exact 10MHz reference with only integer multipliers and dividers (10 MHz×63/88/455/525 - see en.wikipedia.org/wiki/NTSC) \$\endgroup\$ Sep 29, 2020 at 22:16
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    \$\begingroup\$ @KevinWhite There is some info on 29.97 Hz history here: videocide.com/glossary/drop-frame-timecode \$\endgroup\$
    – Dan M.
    Oct 1, 2020 at 10:54

Consider what a 60Hz sine wave looks like and when a light bulb lights up.

The light bulb is basically a resistor so lights up the most when the sinusoid is at its peaks which happens twice per cycle (the positive peak and negative peak). Since there are two of these per cycle there are 120 of them per second in a 60Hz sine wave running through a light bulb makes it flash at 120Hz.

But a monitor refreshes at 60Hz because it was made that way.


Whoa. Hertz does NOT mean cycles per second. Wherever did you get the idea that it was 'cycles' - only joking.

It's a real problem that when the SI system decided on the Hertz they didn't say what thing was being measured/counted over the period of time. This lead to various problems between random arrival things and regular or 'cyclic' events.

There was a note in the SI specification that all the ionising radiation units had to be created specially because of the issue.

Further, a default periodic unit for SI was the radian, so you (we) should be talking Angular Frequency, or Cyclic Frequency, (or event frequency) to try to clarify to readers just what is being counted, which lies at the heart of the 60 (cy) Hz mains frequency and the potential 120 (light pulse) Hz frequency.

In the older 8th Ed of the SI units [updated in 2014] it was the section "Units with special names and symbols; units that incorporate special names and symbols" http://www.bipm.org/en/publications/si-brochure/section2-2-2.html but it's now (new edition) all one pdf!

  • \$\begingroup\$ Honestly, none of this makes sense to me. 1) Hertz is a clear and simple SI unit of frequency which works fine with the rest of the SI system. Calling it cycles per second is quite reasonable - one cycle denotes the duration of the repeated event. 2) Radian is not a periodic unit, it's a unit for measuring angles. Perhaps you meant radians per second, in which 60 Hz would directly translate to 120*pi rad/s without giving any additional information over 60 Hz. 3) Also, what are "ionising radiation units"? \$\endgroup\$
    – vgru
    Oct 29, 2020 at 9:01
  • \$\begingroup\$ @Groo My answer implicitly says that the question is correct about the confusion (about "Hertz"), and that the standards make it worse. Hertz is horrible and subject to many notes in the standard. Historically it was always things like "cycles per second", or "pulses per second", which always said what was counted. Now they omit that vital information. Radian is a 'cyclic' unit (repeats every 2.$pi. Radioactivity (Becquerels) is an ionising radiation (one of quite a few) and has effective units of "Hz", in this case counting "one nucleus decay". \$\endgroup\$ Oct 30, 2020 at 8:32
  • \$\begingroup\$ sorry I guess I didn't understand the point you were making. \$\endgroup\$
    – vgru
    Oct 30, 2020 at 13:03
  • \$\begingroup\$ 1. You're equivocating cycle as in "iteration" and cycle as in "circular". 2. Also, Hertz DOES mean cycles per second. I don't know if you're sarcastic or not, but the first line does not help. 3. And you're not really answering the misconception in the question. \$\endgroup\$
    – Eric
    Jan 2, 2021 at 6:08
  • \$\begingroup\$ @Eric. Under SI, the Hertz has no formal connection to the 'cycle'. The Hertz is just s^-1. It's been a bit of a merry go round for many years. The original question is about the consequences of the different misunderstandings. \$\endgroup\$ Jan 3, 2021 at 11:18

Power is V^2/R (barring impedance effects). So if you have a voltage source of cos(60T), the power will be cos^2(60T)/R, which is equal to (1+cos(120T))/2R, which has a frequency of 120 Hz. In an AC circuit, the power cycles at a frequency twice that of the voltage.

Remember that the power is current times voltage, and current is proportional to the voltage. So when the voltage is at its most negative, the current is also at its most negative, and so the power is at its most positive. So for every peak in the voltage graph, there are two peaks for the power graph: one for when the voltage is most positive, and one for when it is most negative.

  • \$\begingroup\$ This is basically the most realistic answer. The lightbulb acts as a rectifier as far as utilized power/energy is concerned, that's why the "periodic event" where the filament is heated repeats at 120 Hz. \$\endgroup\$
    – vgru
    Oct 29, 2020 at 9:09
  • \$\begingroup\$ One other noteworthy item is that an old fashioned filament incandescent bulb produces light because the filament is very hot. The actual filament temperature remains quite high (> 2000 K) despite the current cycling, as there is enough thermal mass in the tungsten wire to maintain the incandescence. So a standard tungsten bulb doesn't flicker at all. (Old-style fluorescent bulbs, on the other hand, do.) \$\endgroup\$
    – torek
    Oct 30, 2020 at 2:04

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