NDA (II) 2023 General Ability Test · Question 132 of 146

Correct — B, Sound is a mechanical wave and longitudinal in nature. Two independent labels are being tested and both must be right. Sound is mechanical because it is carried by the particles of a material medium pushing on one another; take the medium away and the sound stops, which is why an electric bell sealed in an evacuated jar falls silent while its light still reaches you. Sound in air is longitudinal because the air particles oscillate back and forth along the same line the wave travels, bunching into compressions and spreading into rarefactions. Nothing moves sideways. An electromagnetic wave is the opposite on both counts — it needs no medium and its fields oscillate across the direction of travel.

• (a)Sound is an electromagnetic wave and transverse in nature — Wrong on both counts. Sound cannot cross a vacuum, so it is not electromagnetic, and its air particles oscillate along the direction of travel rather than across it.
• (c)Sound is a mechanical wave and transverse in nature — The first half is right and the second is not, which is what makes this the strongest decoy. Sound is indeed mechanical, but in a gas the only restoring force available is pressure, and pressure can only push along the line of travel. A gas has no shear stiffness, so it cannot carry a sideways oscillation.
• (d)Sound is an electromagnetic wave and longitudinal in nature — The second half is right and the first is not. Sound is longitudinal, but calling it electromagnetic would mean it needs no medium and travels at the speed of light, neither of which is true. Sound moves at roughly 340 m/s in air and stops dead in a vacuum.

Waves split two ways. By what carries them, they are either mechanical — needing a material medium, like sound, water waves and seismic waves — or electromagnetic, needing none, like light, radio and X-rays. By how they oscillate, they are either longitudinal, with particle motion along the direction of travel, or transverse, with particle motion across it. Sound in air is mechanical and longitudinal; light is electromagnetic and transverse. Both labels are independent, and an examiner can test them separately or, as here, together in a single sentence.

A four-option question of this shape is really two true-or-false decisions multiplied together, and the fastest route is to settle them one at a time. First ask whether sound can cross empty space; it cannot, so every option calling it electromagnetic goes. That leaves two. Then ask which way the air moves as a sound passes; along the line of travel, as compressions and rarefactions, so the transverse option goes too. One honest refinement is worth carrying beyond the exam. The question is careful to say in air, because in a solid sound can also travel as a transverse wave — a solid resists shear and a gas does not. That is why earthquakes generate both longitudinal P-waves and transverse S-waves through rock, while only the longitudinal kind crosses the liquid outer core.

• Sound needs a material medium and cannot travel through a vacuum, which is what makes it a mechanical wave.
• In air, the particles of the medium oscillate along the direction of travel, producing compressions and rarefactions — the signature of a longitudinal wave.
• Electromagnetic waves such as light need no medium and are transverse, with electric and magnetic fields oscillating perpendicular to each other and to the direction of travel.
• In solids, sound can travel as a transverse wave as well, because a solid resists shear; a gas cannot support such an oscillation.

• Choosing the option whose first half is right without checking the second half.
• Forgetting the qualifier 'in air' and arguing that sound can be transverse, which is true only in solids.
• Treating sound and light as the same kind of wave because both are described by frequency and wavelength.

Asked as a compound true-or-false item — two independent properties combined into four sentences, only one of which is right on both.

• practice — not a real PYQ

  An electric bell ringing inside a sealed jar becomes inaudible as the air is pumped out because

  • (a)the bell stops vibrating in a vacuum
  • (b)sound needs a material medium to propagate
  • (c)the frequency of the sound falls below the audible range
  • (d)the glass of the jar absorbs the sound

  Answer(b) sound needs a material medium to propagate — with no particles left to pass the disturbance on, no sound reaches the observer.

• practice — not a real PYQ

  Which one of the following is a transverse wave?

  • (a)Sound wave in air
  • (b)Ultrasound in water
  • (c)Light wave
  • (d)P-wave of an earthquake

  Answer(c) Light wave — its electric and magnetic fields oscillate across the direction of travel; the other three are longitudinal.