How does the triac dimmer work?

All dimmer commonly mounted in the wall or for desk lamp is the Triac dimmer, The triac is the key to dimming. This dimmer component actually turns light on and off very rapidly - 120 times (for 60Hz mains voltage, For 50Hz mains voltage is 100 times) per second. This technology is illustrated below.

The longer the light is ON versus OFF (example A) the brighter the light output. By the same logic the longer the light is OFF versus ON (example B) the lower the light output.