AC loads are everywhere around us because at least home appliances are supplied with the mains power. Therefore, we always face the situations that we want to have full control (dimming) over an AC load such as a lamp, a motor, vacuum cleaner … etc. 

We should know that controlling an AC load is not identical with a DC load. So we should use different electronic circuits for this purpose.

The figure-1 shows the mains sinusoidal wave with the frequency of 50Hz (sometimes 60Hz). To build a dimmer, the zero crossing points (the points where the wave changes its polarity) are important. To grab these points, we have to use a zero crossing detector. The figure-2 demonstrates the schematic diagram of the whole circuit.

More details please check https://www.pcbway.com/blog/technology/How_to_build_an_isolated_digital_AC_dimmer_using_Arduino.html