Complex Numbers - Remix
This Khan Academy video was shared through the OER commons: https://www.oercommons.org/courses/complex-numbers-3
The video provides an introduction to complex numbers.
As a follow-up to the video I would use the Desmos Activity by Luke Walsh to help students visualize imaginary numbers. https://teacher.desmos.com/activitybuilder/custom/56094c8558e4a93806b14a58