For 1. The filament in the bulb heats up, and it takes longer than 1/60th of a second to cool down again. The light it gives off depends on it's temperature - so it does not dim right away. You can see this when you switch off a very bright light (don't look at it) the illumination takes a short while to fade away. Lights that use other methods that respond faster to changes in current show an appreciable flicker.
For 2. You get the impedence from the relationship between voltage and current, not the voltage alone.