Investigation of physical properties of an object

A system for investigating physical properties of an object, the system comprising:

• an acoustic transducer apparatus arranged to apply to the object acoustic vibration localised in two or three dimensions in a region in the object;
• a transmitter arrangement arranged to illuminate the object with an illuminating electromagnetic wave having a frequency in a range extending down from 30 THz simultaneously with the application of acoustic vibration, the vibration direction of the acoustic vibration having a component parallel to the propagation direction of the illuminating electromagnetic wave so that the acoustic vibration of the object in the region generates a scattered electromagnetic wave including Doppler components shifted from the frequency of the illuminating electromagnetic wave by frequencies of the acoustic vibration and multiples thereof;
• a receiver arrangement arranged to receive the scattered electromagnetic wave generated in the region;
• and
• a signal processing apparatus arranged to derive, from the received scattered electromagnetic wave generated in the region, data representing at least one characteristic of the Doppler components.