http://www.artfulartsyamy.com/2012/01/lesson-plan-op-art-spheres-easy-way.html
https://www.pinterest.com/moniqueharrand/teaching-op-art/
http://www.incredibleart.org/lessons/elem/patti-opart.htm
https://www.theartofed.com/lesson-plans/op-art-optical-illusions/