Fresnel lenses are found in all kinds of things from the lenses and light modifiers we use in photography and filmmaking to lighthouses that guide ships at sea. The lens was developed by French physicist Augustin-Jean Fresnel, after whom it was named, specifically for use in lighthouses. It allows for the construction of huge aperture and short focal length lenses but using less material than more traditional lens optics.
How exactly do they work, though? In the video above, David Willey explains how a Fresnel lens functions and why it is the shape that it is using a simple blackboard model and some magnetic strips. But essentially, Fresnel lenses are just regular lenses with big chunks taken out of them.