When an LED is flashing at 2 times a second, the human eye can perceive the ON and OFF state. We can clearly see when the ON state is and when the OFF state is.

When it is flashing 10,000 times per second, we see the LED in the ON state only.

Why is it that when the LED is flashing at 10,000 times per second, it is the ON state that is visible to the human eye but not the OFF state? Why does the ON state seem to have precedence over the OFF state?

Why don't we see the LED as OFF while it is flashing at 10,000 times per second? If the time periods of both states are the same, why does the LED seem to be ON all the time?

Why does nature choose to show us the ON state only all the time? Why does not nature choose to show us the OFF state only all the time?

( Assume the duty cycle is 50%. )

  • $\begingroup$ There's no reverse flashlight, something you can "shine" into a room and make everything dark. Darkness is the absence of light, "lightness" is not the absence of darkness. Put another way, when you stare into a bright light and look away, you still you see the light, there's no opposite of this $\endgroup$
    – eps
    Commented Mar 29, 2022 at 0:05

2 Answers 2


The phenomena you are referring to is called persistence of vision and is actually quite complicated. For instance, the rate the LED needs to blink to be interpreted as continuously on changes with how fast it is moving across your line of sight and where in your field of view it is. The short answer to your question is that the way your eye turns a photon into a notion in your mind is through a series of neurons starting with photoreceptor cells (the rods and cones) how quickly they can activate and deactivate depends on chemicals within the cell [1]. The longer and more correct answer to your question lies within the functioning photoreceptor cells. To give a brief (and likely wrong) explanation, consider experiments performed on photoreceptors extracted from turtle eyes. These experiments show that the signal leaving the photoreceptor persist for a time longer than the light stimulus itself [2]. If the sustained signaling from the previous flash begins to overlap with the current flash, it is easy to understand how the light would seem continuously on in your mind.

[1] Pearring, Jillian N., et al. "Protein sorting, targeting and trafficking in photoreceptor cells." Progress in retinal and eye research 36 (2013): 24-51. https://doi.org/10.1016/j.preteyeres.2013.03.002

[2] Baylor, D. A., and A. L. Hodgkin. "Changes in time scale and sensitivity in turtle photoreceptors." The Journal of Physiology 242.3 (1974): 729-758. https://doi.org/10.1113/jphysiol.1974.sp010732

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    $\begingroup$ This is true for any kind of sensor, be it biological or technical. If the interval between flashes is less than the response time of the sensor, the sensor cannot distinguish between flashes and steady emission. $\endgroup$
    – John Doty
    Commented Mar 28, 2022 at 16:34
  • $\begingroup$ @JohnDoty But why do we see it as being ON all the time? We might as well see it as OFF all the time. $\endgroup$ Commented Mar 28, 2022 at 16:52
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    $\begingroup$ @MarsSojourner It doesn't look like ON all the time. If it's pulsing half on, half off, it looks dimmer than it does if it's on all the time. $\endgroup$
    – John Doty
    Commented Mar 28, 2022 at 16:54
  • $\begingroup$ @MarsSojourner: that is true. Obviously the level where the perceived intensity is still above threshold is (much) lower than 0.5 (assuming that you are talking about a 50% duty cycle). If you reduce the duty cycle to something very small, you might eventually find a level where you cannot perceive any brightness anymore (which is of course, adaptive), but there is still physical intensity (in the sense of emitted photons). BTW the eye (if properly adapted to darkness) can be extremely sensitive to up to a few photons, as far as I remember. $\endgroup$
    – oliver
    Commented Mar 28, 2022 at 16:58
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    $\begingroup$ @user1079505 Sure, there are complications, but I think of dead time and pileup as properties of the measurement system, not the sensor. With a photoelectric sensor, you may record a time average of the photocurrent to obtain the optical power without deadtime or pileup difficulties, losing the benefits of photon-by-photon detection. $\endgroup$
    – John Doty
    Commented Mar 29, 2022 at 13:06

Chris has a great explanation in their up to the physics of the chemistry in the eyeball. I would like to add to that a psychological consideration that might help close the loop for you. Our eye never sees the LED "on" or "off" Indeed, it doesn't see an LED at all. It sees a bunch of light coming from that direction, and that light goes through the physics Chris describes.

"On" and "Off" are psychological constructs you have based on understanding how the world works, and LED's place in that world. In this case, it is likely that your brain realizes that the presence of an 'on' LED is important, and ensures your worldview includes one.

You can change the duty cycle to identify a point where your brain feels it is valid to also include the presence of an 'off' LED. This transition can feel quite uncomfortable, because you know the LED should be either on or off, but it feels like both. You could also change the current going through the LED to achieve the same effect. You can get to a situation where "off" describes what the eye sees sufficiently well, and your brain starts ignoring the fact that there's some brightness to it. This practically happens with white LEDs, which rely on a UV LED and a phosphor that glows brightly white in the presence of UV light. Typically, when the light is turned off, we observe it as "off." However, if you look at the phosphors, and really try to see them, you'll see that they're still glowing. They have a rather slow decay rate that persists for a long time.

I believe this is highly related to how our brain builds our worldview when driving into the sun. Often the sun obscures our vision completely, but we feel the need to "see through" the sun, because we know there's some really important cars out there. If I wasn't driving, and there were no important cars out there, I may not feel the need to observe that there's anything behind that glare.


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