A question on thyristors and phase control
Here is few factors come together. Flickering frequency is 120 or 100 Hz. Human eye can see it only by side view, not when you look straight. Second,  voltage changes, but current amplitude doesn't follow voltage exactly, because resistance of filament not constant, it depends of temperature. So, filament heats, current decrease. Wattage amplitude smaller. And third, the filament still hot and glows some time after voltage became zero. LEDs fixtures is different story. They fed by DC and brightness regulated using PWM on higher frequency.