Focal length of convex lens (uv method)

Easy understanding of the biconvex lens and related focal length determination.
Boya mics
https://amzn.to/3dMjfdB
Lenovo Tablets https://amzn.to/3sMAFuN
https://amzn.to/3njKoHP (infrared digital thermometer)
https://amzn.to/3esAFek (Philips home theatre)
https://amzn.to/3vfO8wS (Omron Digital BP machine)