Ultrasound imaging creates a picture of something that cannot be seen directly, such as an unborn baby in the womb (a foetus), or faults and defects inside metals.
These uses rely on what happens when ultrasound waves meet the boundary between two different materials.
The best known example of the use of ultrasound is medical imaging, to 'see' inside a body.
An ultrasound scanner is simply run over the skin to obtain an image of what's inside.
The scanner probe has a built-in transducer that directs ultrasound pulses down into the body.
As the waves travel through the different bones and tissues, some are reflected off a boundary but some travel further to be reflected back up again off another boundary as some parts have different densities.
The probe receives the reflected waves and a computer connected to the scanner uses them to draw an image on a screen.
Scans of foetuses (unborn babies developing in the womb) are made this way and are used, for example, to measure the diameter of the head of a foetus so that growth can be monitored.
Ultrasound imaging also helps to diagnose problems with the:
The best known non-imaging medical use of ultrasound is the breaking of kidney stones.
The vibrations caused by the ultrasound shake apart the kidney stones, breaking them up.