Question

What would happen to brightness of bulb when a dielectric slab is introduced to a capacitor connected to bulb and in ac source?

Answer 1

➡I agree the bulb will dim for a short period of time, during which time the cap is charging. Once charged no further energy can be stored in the cap so full energy goes to the bulb. As the current swings the other way the cap will discharge so the bulb will go from bright to dim.
Then the cycle begins once the current swings the other way again.
I think to an observer the bulb will go from a dim to bright to dim and flicker.