First, its a heating process. So when current passes in one direction, it gets heated up. So it emits light. Before , it starts cooling to a low temp , current passes again, in the other direction, thereby heating it again.
Second, we wud not b able to notice such a rapid change due to the high freq. Our eye can only differentiate movements seperated by atleast 1/16 of a second. This is called continuity of vision or some thin. Forgot.... : ) (U cant differentiate btwn the blades of a fan rotatin at high speed)
So actually , the bulb mite be blinking rapidly but v r not able to notice it.