Concave and Convex Lenses
Dr Mark Fernihough
Move the tip of the "Object" arrow to move the object. Move the point named " Focus' " to change the focal length. Move the point named " Focus' " to the right side of the lens to change to a concave lens.
Regular octagon out of a square
UpperSum and LowerSum: Integral
Vivek_Circles and Rectangles