For visiable artifacts a flash is the entire on state phase of a signal. In the case of an AC signal this is From 0 volts to positive peek, back down to 0 volts, and from 0 volts to negative peek and back up to 0 volts. So there are two 'flashes' per AC phase of an incadescent bulb. In the case of a 50hz lines this is 100hz and 60hz line it's 120hz.
I don't know about any of you but I can spot a car with LED brake/turn lamps from 50 feet away by noticing that they don't dim/brighten like normal incadescents do. Since mains AC is at least 50hz this dimming isn't noticeable to the human eye. Human eye can see flicker at 50-60hz easily when it's abrupt on/off but not when it's only the slight dimming effect of an AC bulb, so the human eye is never aware of any kind of flicker, it's just too small. It does however exist, as someone mentioned using a photo diode. You can also use a Cadmium sulfide cell as a resistor in a simple audio oscilator and amplify it a lot and you'll be able to hear the line hum from a light bulb.