A sensor comprises a beam 1, e.g. of silicon, which resonates at a frequency dependent on the force imposed on the beam. Light on line 11 induces resonance of the beam 1 by means of the photothermal effect. The light reflected from the beam 1 is amplitude modulated at the resonance frequency, and returns along line 11. It is reflected by semi-reflecting plane mirror 17 onto lens 19 which focusses it onto photodetector 20. Photodetector 20 produces an output voltage modulated at the oscillation frequency, and is thus representative of the force imposed on beam 1.