Huygens' principle

(Christian Huygens, 16291695, Dutch mathematician and astronomist), a principle stating that the surface of a radiating source may be considered to be made up of an infinite number of point sources. The principle explains how a flat ultrasound transducer can transmit a narrow ultrasound beam which in the near field is confined to the dimensions of the transducer surface (Fig.1). Spherical wavelets emitted from numerous point sources on the transducer surface interfere to form a narrow, slightly converging beam of ultrasound in the near field. The wavefronts in the beam are nearly parallel (actually slightly concave as long as the beam is slightly converging). As can be seen from the figure, a prerequisite for this interference is that the transducer surface be much larger than the ultrasound wavelength.

HJS