The small sources of sound producing the V shaped wavelets are known as ______.

• A. Huygens sources
• B. PZT crystals
• C. Transducers
• D. Ultrasound

Answer: (A)

Huygens sources come from the idea that every point on a wavefront acts as its own little source, emitting wavelets. This is the Huygens-Fresnel principle: the new wavefront is the envelope of all those secondary wavelets. In diagrams, these small sources produce the V-shaped wavelets as they radiate outward, and their combined effect forms the advancing front. In ultrasound, the transducer is made of many small elements, and each element behaves as a Huygens source when it emits sound. The other terms—PZT crystals refer to the piezoelectric material inside the element, the transducer is the whole device, and ultrasound is the type of wave—do not name the concept of these secondary wave sources.