Which one of the following statements is not correct ?
- (a)Ultrasonic waves cannot get reflected, refracted or absorbed.
- (b)Ultrasonic waves are used to detect the presence of defects like cracks, porosity, etc. in the internal structure of common structure materials.
- (c)Ultrasonic waves can be used for making holes in very hard materials like diamond.
- (d)Ultrasonic waves cannot travel through vacuum.
Correct — A, the claim that ultrasonic waves cannot be reflected, refracted or absorbed. Ultrasound is ordinary sound, only at a frequency above about 20,000 Hz, so it obeys every rule sound obeys: it reflects at a boundary, it refracts when the speed changes from one medium to another, and it is progressively absorbed as it travels. Reflection is not merely possible but is the whole basis of the technology — a bat's echolocation, a ship's SONAR, a hospital scan and the flaw detector in statement (b) all work by listening to a reflected ultrasonic pulse. If statement (a) were true, none of the other three statements could be.
- (b)Ultrasonic waves are used to detect the presence of defects like cracks, porosity, etc. in the internal structure of common structure materials. — This is correct, and it is standard industrial practice. Ultrasonic flaw detection sends a pulse into a casting, a weld or a rail and reads the echo returned by an internal crack or void, without cutting the component open.
- (c)Ultrasonic waves can be used for making holes in very hard materials like diamond. — Also correct. In ultrasonic machining a tool vibrating at ultrasonic frequency drives an abrasive slurry against the workpiece, chipping away material a little at a time. It is the usual method for drilling brittle, very hard materials that resist conventional cutting.
- (d)Ultrasonic waves cannot travel through vacuum. — Correct as well. Ultrasound is a mechanical wave and needs particles of a medium to pass the vibration along, so like all sound it dies out in vacuum.
Ultrasound is sound above the human hearing limit of roughly 20,000 Hz. Because frequency is high, wavelength is short — a few millimetres or less — and short wavelength buys two things: the beam can be aimed narrowly instead of spreading, and it reflects from small features that longer waves would simply flow around. That is why ultrasound is the tool of choice wherever something small has to be located without being seen, whether an internal crack, an organ, a shoal of fish or the sea bed.
Notice how the statements police each other. Three of them describe things ultrasound does, and the fourth denies that it can do any of them, so the odd one out can be found without recalling a single application. That internal-consistency check is worth practising: in a not-correct item, look for the statement that contradicts the others rather than trying to verify each one in isolation.
- Ultrasound is sound of frequency above about 20,000 Hz and behaves exactly like audible sound.
- Reflection of ultrasound is the basis of SONAR, medical imaging and industrial flaw detection.
- Ultrasonic machining uses a vibrating tool and abrasive slurry to cut hard, brittle materials.
- Being mechanical, ultrasound cannot cross a vacuum.
- Ultrasonic cleaning uses cavitation in a liquid bath to lift dirt from delicate parts.
- Assuming ultrasound is a different kind of wave from ordinary sound; only the frequency differs.
- Believing ultrasound can travel through vacuum because it is used in imaging.
- Confusing ultrasonic scanning with X-ray imaging, which uses electromagnetic radiation.
Ultrasound is asked as a not-correct statement item on its properties, as a SONAR calculation, or as an application-matching question.
SONAR is a device that is used to measure the distance of underwater objects by a ship. Which of the following types of waves does it use for this purpose?
- (a) Infrasonic waves
- (b) Sound waves in audible range for human beings
- (c) Ultrasonic waves
- (d) All of the above
Answer(c) Ultrasonic waves
Direct counter-example to statement (a): SONAR exists only because ultrasound does reflect.
Directions : The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below. Statement I : Sound wave cannot propagate in vacuum. Statement II : Sound waves are elastic waves and require a medium to propagate.
- (a) Both the statements are individually true and Statement II is the correct explanation of Statement I
- (b) Both the statements are individually true but Statement II is not the correct explanation of Statement I
- (c) Statement I is true but Statement II is false
- (d) Statement I is false but Statement II is true
Answer(a) Both the statements are individually true and Statement II is the correct explanation of Statement I
Establishes statement (d) of this question, and by the same reasoning that ultrasound is ordinary sound.
- practice — not a real PYQ
Ultrasound is preferred over audible sound for detecting small internal cracks in a metal casting mainly because
- (a)it travels faster in metals
- (b)its short wavelength allows it to reflect from small defects
- (c)it is not absorbed by any material
- (d)it can travel through vacuum
Answer(b) its short wavelength allows it to reflect from small defects — resolution improves as wavelength falls.
- practice — not a real PYQ
Which one of the following does not depend on the reflection of ultrasonic waves?
- (a)SONAR depth sounding
- (b)Ultrasound scanning of the abdomen
- (c)Detection of cracks in a railway rail
- (d)Transmission of a television signal
Answer(d) Transmission of a television signal — that uses electromagnetic waves, not reflected ultrasound.