Weaving Illusions

Is Rainbow an illusion? Is mirage an illusion? Is illusion caused always by optical refraction or can it be a result of an individual’s visual perception based on how the brain and senses process the signals? Like grouping the dots of the same colour or converging the lines or the picture where we train our brain and vision to see the old woman and young girl in the same image?
 
Questions for which there is more than one answer. But isn’t creating an illusion and understanding how it works a better way to learn about it rather than theorizing it?
 
In our school weaving program, the children have an exercise on creating an illusion by weaving lines of black & white in a particular order and understanding how illusion is created by the arrangement of lines.