NDA (II) 2017 General Ability Test · Question 126 of 150

Correct — B, ultrasonic waves. A bat navigates by echolocation: it emits short bursts of sound above the human hearing limit of about 20,000 Hz, and its ears pick up the echo bouncing back off obstacles and prey. The delay between the cry and the echo gives the distance, and the direction of arrival gives the bearing. Ultrasound is the right tool for the job because its wavelength is only a few millimetres, and a wave reflects cleanly only from objects comparable to or larger than its own wavelength — which is how a bat can detect a wire or a flying moth in total darkness.

• (a)Infrasonic waves — Infrasound lies below about 20 Hz, at the other end of the spectrum. Its wavelength runs to many metres, so it bends around small obstacles instead of reflecting from them, making it useless for locating a moth. Elephants and whales use infrasound, but for long-distance communication, not for obstacle detection.
• (c)Radio waves — Radio waves are electromagnetic, produced by oscillating electric charges and detected by antennas. No animal organ generates or receives them, and a bat's larynx and ears are built for sound. Radio-based obstacle detection is radar, a human invention, not biology.
• (d)Microwaves — Microwaves are also electromagnetic, just at shorter wavelength than radio. Same objection applies — no biological transmitter or receiver exists for them, and they would pass through or be absorbed by soft tissue rather than being usefully echoed back by an insect.

Sound is classified by frequency: infrasonic below roughly 20 Hz, audible from about 20 Hz to 20,000 Hz, and ultrasonic above 20,000 Hz. All three are mechanical waves and need a material medium. Echolocation is the biological use of ultrasound — the animal emits a pulse and times the returning echo. Bats and dolphins both do it; the same physics underlies the ship-borne SONAR and the medical ultrasound scanner.

The trap here is that all four options are waves, so a student who has not sorted them into mechanical and electromagnetic families may guess. Sort them first: infrasonic and ultrasonic are sound, radio and microwave are electromagnetic. Then apply the resolution rule — the smaller the object you want to see, the shorter the wavelength you need. Among the two sound options only the ultrasonic one has a wavelength short enough to bounce off an insect.

• Ultrasound is sound of frequency above about 20,000 Hz, the upper limit of normal human hearing.
• Bats and dolphins locate objects by echolocation — emitting a pulse and timing the returning echo.
• Infrasound is below about 20 Hz; elephants and whales use it for long-range communication.
• Radio waves and microwaves are electromagnetic and travel through vacuum; sound of every frequency needs a medium.
• The same principle drives SONAR at sea and the ultrasound scanner in a hospital.

• Treating radio waves and microwaves as if an animal could produce them.
• Confusing infrasonic with ultrasonic — the prefixes point in opposite directions.
• Assuming ultrasound travels faster than audible sound; the speed is set by the medium, not the frequency.

Ultrasound appears either as this bat question, as a SONAR item, or as a not-correct statement item about the properties of ultrasonic waves.

• practice — not a real PYQ

  The frequency of ultrasonic waves is greater than

  • (a)20 Hz
  • (b)200 Hz
  • (c)2,000 Hz
  • (d)20,000 Hz

  Answer(d) 20,000 Hz — the upper limit of normal human hearing, above which sound is called ultrasonic.

• practice — not a real PYQ

  Which one of the following pairs uses the same physical principle as a bat finding its way in the dark?

  • (a)Radar and radio telescope
  • (b)SONAR and medical ultrasound scanning
  • (c)Microwave oven and mobile telephone
  • (d)Infrared camera and night-vision goggles

  Answer(b) SONAR and medical ultrasound scanning — both send an ultrasonic pulse and time the returning echo.