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Question 1 of 5 · Waves & Sound
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Sound cannot travel through:

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Answer: Option C — Vacuum

Detailed Solution

Sound is a mechanical wave. It is produced by vibrating objects and travels as a series of compressions (regions of high pressure, where particles are close together) and rarefactions (regions of low pressure, where particles are spread out) through a medium. In air, sound waves are longitudinal waves: the particles of the medium vibrate back and forth in the same direction in which the wave travels.

Because sound needs particles of a material medium (solid, liquid or gas) to pass on the vibrations, it cannot travel through a vacuum, where there are no particles. This can be shown by the "bell jar experiment": an electric bell is placed inside a glass jar connected to a vacuum pump. As the air is pumped out, the sound of the bell becomes fainter and fainter, even though the bell can still be seen ringing. This experiment was first carried out by Robert Boyle in 1660.

This is also why there is no sound in outer space. Astronauts on the Moon or in space communicate using radio waves, which are electromagnetic waves that can travel through a vacuum.

The speed of sound depends on the medium:

  • It is fastest in solids, slower in liquids and slowest in gases, because particles are packed more closely and are more tightly bound in solids.
  • Approximate speeds: air (at 20°C) about 343 m/s (about 332 m/s at 0°C); water about 1,480 m/s; steel about 5,960 m/s.

The speed of sound in air increases with temperature (by about 0.6 m/s per °C) and with humidity. It does not depend on pressure at constant temperature.

Since light travels much faster than sound (about 3 × 10⁸ m/s), we see a flash of lightning before we hear the thunder.

Objects moving faster than the speed of sound are called supersonic. The ratio of an object's speed to the speed of sound is its Mach number. A supersonic aircraft produces a "sonic boom".

Why not the other options?

  • ✗Water: sound travels about four times faster in water than in air.
  • ✗Steel: sound travels very fast in solids such as steel.
  • ✗Air: sound travels through air at about 343 m/s.

Quick tip

Remember: Sound = mechanical, longitudinal wave, needs a medium (cannot travel in vacuum; bell jar experiment); speed: solids > liquids > gases; air about 343 m/s; light needs no medium.

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Waves & Sound — Preparation

This question tests your understanding of Waves & Sound, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.

This topic covers waves and their types (transverse and longitudinal), the properties of sound, the speed of sound in different media, echo, reverberation, the Doppler effect, pitch, loudness and the decibel scale, and infrasonic and ultrasonic sound and their uses. Questions often ask in which medium sound travels fastest (solids) and why it cannot travel through a vacuum, the human audible range (about 20 Hz to 20,000 Hz), the instrument used to detect underwater objects (SONAR), and the effect that explains the changing pitch of a passing train's horn (the Doppler effect). Practise the waves and sound questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.

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