= Solution
In <contact-mode atomic force microscopy>, \b[a sharp tip on a compliant <cantilever> remains in contact with the surface while a feedback loop controls its normal deflection]. A laser reflected from the <cantilever> onto a position-sensitive detector converts bending into an electrical error signal. With calibrated deflection $d$ and stiffness $k_c$, the normal <force> is approximately $F=k_cd$ under the stated contact convention.
A piezoelectric scanner raster-scans the sample or tip. The controller adjusts vertical position to maintain the chosen deflection setpoint; that vertical displacement is recorded as topography. The right panel of the microscopy diagram identifies tip, <cantilever>, optical deflection readout, controller and vertical actuator.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2018/paper-1-microscopy.png]
{title=Original instrument diagrams; the right panel shows contact-mode <atomic force microscopy> and the recorded vertical feedback signal}
The result is a probe-accessible surface, affected by finite tip shape, sample deformation, adhesion and feedback bandwidth. Lateral friction can distort or damage a soft surface, so constant deflection is not a guarantee of unchanged specimen structure.
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