Dimming an AC light in this way is not easy. Timing is critical.
Unlike DC, it's not just a case of varying the amount of time that the light is on for, but it is a case of chopping and modifying the AC waveform.
It's also quite dangerous playing with mains AC, so I cannot condone this in any way.
You should look at (and thoroughly understand) this site: Voltage Controlled AC Light Dimmer