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Whether a flash of light is visible comes down to a fundamental concept in visual neuroscience known as temporal summation or generally Bloch's Law.
Our eyes don't process light instantaneously, they integrate light over a short window of time (around 10 to 100 milliseconds, depending on whether you are looking through your central or peripheral vision).
Bloch's Law states that for very short durations, the perceived brightness of a flash depends on the total number of photons hitting the retina during that time window, not the duration itself.
So you can flash a standard light source like a normal LED very quickly, like at 1 microsecond or faster, it will be completely invisible. The flash was so short that the total number of photons entering your eye fell way below the minimum threshold required to trigger your photoreceptors and send a signal to your brain.
inversely you can increase the intensity of the light enough that it will always be seen regardless of the duration of the pulse, a high-powered laser pulsed for a femtosecond will remain visible.