Answer
At this low frequency, the filament has enough time to cool down and become noticeably dim during the low-current portions of the cycle, causing the light to flicker.
Work Step by Step
At a frequency of 5 Hz, the tungsten filament in the old-fashioned light bulb will hit maximum power 10 times per second. (The current reaches its maximum magnitude twice during the period of 1/5 of a second, once for the current flow in each direction.)