Here is a YouTube video about With Complex Numbers Multiplying by j is Rotation, and is part of a growing collection of videos I have found to be useful. The reason is multiplication or division with complex numbers used in DSP computations and understanding science related to software defined radio.
This tutorial, "With Complex Numbers Multiplying by j is Rotation" is free education you can watch, which will help you deepen your knowledge and be better prepared for licence exams, schoolwork, or operating radio equipment.