Inside a simple vintage disco light. (Single Derby.)

This is one of the simplest disco style lighting effects of all. No movement (other than the fan) - just a flat beam in three coloured sections that fans out from the front of the unit.
The efficiency is horrific like most of the lights from that era. High power LEDs were not available back then so the lights used tungsten or discharge sources. In this case it's a 300W halogen source of which only a tiny percentage of its light output is used.
Lights like these tended to be controlled by universal switch packs that could either be used to toggle the light on and off or put it in a simple group and then step through them, either on a timer or to the beat of the music.

The modern version of this usually has a panel of LEDs that rotates back and forth while chasing the LEDs. It's smaller, projects a lot of beams and runs almost cold.

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