In atomic force microscopy, a sharp tip at the end of a flexible cantilever beam is scanned across the sample surface while the tip is interacting with the surface. Forces acting on the tip bends the cantilever in a detectible amount. Deflections of the cantilever can be measured by reflecting a laser beam from the backside of the cantilever that falls onto a quadrant photodetector (see the illustration on the right). As the tip is scanned across the sample surface, deflections of the cantilever beam is used to map the surface topography.