Say you had a laser that pulsed faster than your eye could perceive it, so that it appeared to fire constantly. Imagine that in between each pulse the laser changed to every color of the rainbow for an equally spaced amount of time and changed color almost instantaneously, so that the color change was basically discrete. All of the colors would have an equal amount of firing time before switching in between each pulse.

What color would the laser appear to be to the naked eye?

Just for clarity, let’s say right after a pulse the color change is red, orange, yellow, blue, purple and then it pulses and then repeats itself.

If it’s merely red since red is always first, let’s say it randomized the color change order after each pulse.

  • 1
    $\begingroup$ Is it intentional you left green out of the color sequence? $\endgroup$
    – The Photon
    Jul 8, 2018 at 15:05
  • $\begingroup$ @ThePhoton No, I chose the colors arbitrarily. $\endgroup$
    – sangstar
    Jul 8, 2018 at 15:10
  • 1
    $\begingroup$ Howe is this different from the random arrival times of photons from the red-blue-green pixels on a screen ? $\endgroup$ Sep 18, 2018 at 21:48

1 Answer 1


The pulses being "ultra fast" plays no role here. So long as the pulse repetition rate (of each color) is above the flicker fusion threshold, each individual set of colored pulses will be indistinguishable from a steady light source to the naked eye.

The actual color perceived will obviously depend on the choices of the colors and their relative intensity, but it will be the same as if you took a bunch of continuous-wave or incoherent light sources of all the individual colors of your pulses and with the corresponding intensities (i.e. with the same global spectrum) and you combined them.


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