With this simple but very clever setup, they managed to measure the movement of the concrete wall just by pushing it with their hands ! Sensitivity is obtained using a laser pointer, and it works with any wall.
And a french version :
Useful links if you want to try it yourself and better understand :
MacIsaac, Daniel L., et Michael Nordstrand. « Demonstrating and Measuring the Flexure of a Masonry Wall ». The Physics Teacher 39, no 4 (avril 2001): 212‑14. https://doi.org/10.1119/1.1367786.
Irene Marzoli, Jean-Michel Courty, Joël Chevrier and Nathalie Lidgi-Guigui made it