The cantilever works as a spring, which converts a force into a deflection.

The attractive or repulsive force between the tip and the sample causes a deflection of the cantilever towards or away from the sample.

The deflection is measured by a laser beam directed at the back of the cantilever. As the cantilever deflects, the angle of the reflected beam changes, and the spot falls on a different part of the photodetector.

The signals from the four quadrants of the detector are compared to calculate the deflection signal.