An optical sensor uses a torsion-mounted thin silicon paddle (1) with magnetically soft material (2) on its back. A light beam falls on the front face and part of the beam is reflected to a photo-diode (4) whose output drives a current in a magnetising coil (5). The magnetic field thus produced rotates the paddle out of the beam, which cuts off the light to the photo-diode. Thus the paddle oscillates with an amplitude which depends on the strain due to the torsion mounting. This modulates the light beam, and if they have different resonances several such sensors can share the same beam with common read-out means.