This allows ultrasound waves to be used for both medical and industrial…

The idea

Because part of a pulse reflects at every boundary and the return time gives the depth, a scanner can build up a picture. In medicine ultrasound is used for prenatal scanning, for imaging soft tissue such as the heart and liver, and for breaking up kidney stones. It is chosen over X-rays for a foetus because it is not ionising, so it will not mutate genes. In industry the same method finds cracks, voids and corrosion inside castings, welds and pipes without cutting them open, which is why it is called non-destructive testing. The limits are real: ultrasound is almost completely reflected at a tissue to air or tissue to bone boundary, so it cannot image the lungs or see through the skull.

What earns the marks

• Naming a medical use, such as prenatal scanning or imaging soft tissue.
• Naming an industrial use, such as detecting cracks or flaws inside metal.
• Stating that ultrasound is non-ionising, so it is safer than X-rays for a foetus.
• Explaining that the depth of each feature comes from the time for its echo to return.

Common mistakes and how to put them right

• Saying ultrasound is safe because it is a sound wave, with no mention of ionisation.

• Claiming ultrasound can image bone and lung tissue as well as it images soft tissue.

• Giving a use without saying which wave property makes it work.

• Confusing ultrasound imaging with X-ray imaging in an application question.

Exam tips

• Pair every use with a reason; a bare list of uses usually scores one mark at most.
• Use the word ionising when comparing ultrasound with X-rays, since that is the safety argument.
• For non-destructive testing, say explicitly that the object does not have to be cut open.