Frequency

I think the idea of a "blink" is a bit of a stretch. The POWER crosses zero 50 (or 60) times per second, but if you divide that into even the tiniest of time slices, it is only AT zero for a fraction of a fraction of an instant. In other words, before the current crosses zero it is at some other measurable level, as it is immediately afterward again. So when we say that the voltage is 120V or 240V, we are really speaking of the RMS voltage, a kind of average between the peak and zero. The peak voltage is actually higher than what we call it; i.e. for a 240V line the peak voltage is actually 339.5V, but when the RMS is calculated the EFFECTIVE voltage is 240V. So just as the utilization in an AC system is never referred to by the peak level, neither is it referred to by its the nadir (zero).

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So is a light bulb "blinking" then? NO. It is "glowing" from the intense heat created by the power flowing through it, which is going up and down constantly. What we PERCEIVE and a constant level of light is in actuality a phenomenon of our eyes called "persistence of vision". Just as an AC machine or heater element cannot respond to a change in current on an immediate basis, neither can the human eye respond to a change in light intensity. We do not perceive the "flicker" until it reaches something as low as 60Hz, but keep in mind that in AC, the "flicker rate" is TWICE the Hz. So on a 50Hz system, the light intensity is changing 100 times per second and in 60Hz, 120 times.

With an incandescent light filiment (something soon to be a relic), the intensity is further dampened by the fact that the temperature in a vacuum (the glass bulb) will not change very quickly, so as the power is "flickering" at 100Hz, the actual light intensitry is varying hardly at all. Fluorescent lighting is actually 'flickering" more because the light intensity is more dependent upon the excitation persistence of the phosphor lining of the tube, which is extremely short in comparison. So the 100Hz flicker is transmitted almost directly as light output. That is the basis of some studies that claim that fuorescent lighting can cause problems for some people. these claims are often based on studies of birds, who seem to be able to detect flickers of 100Hz, leading many aviaries to insist on fluorescent lighting with a minimum of 120Hz. Most accepted studies however have still confirmed that humna eyes cannot perceive flash rates greater than 60Hz.

In short,

The incandescent bulbs emit light because of the heating effect and the filament is not cooled down within the moment of zero point of current, it doesn't blink.

But ….

The fluorescent light really blinks. Though our eyes are not smart enough to catch this blinking, we can notice it if we use a rotating equipment (a ceiling fan) in fluorescent light area. In a room of fluorescent light, start a fan and you will see the fan starts rotating in a direction, then see it is getting stopped, then see it starts rotating in the reverse direction, then again in the forward direction. …. etc. This phenomena is because of the actual blinking of the lights and it is called Stroboscopic effect (which is also used to measure the RPM of a rotating equipment). When the fan's RPM is exactly the multiples of frequency, you will see it stops.

The Stroboscopic effect is very dangerous in the industry where there are many rotating equipment. To eliminate this effect, the fluorescents lights are supplied from three different phases in the same area so that the zero crossing moments are not the same time.

-MS