Some common mediums in which speed of sound waves is measured are mentioned below: 1. Air 2. Steel 3. Copper 4. Water What is the correct increasing order of the speed of sound?

1. Nature of Sound: Mechanical and Longitudinal Waves (basic)

To understand sound, we must first look at how it moves. Sound is a mechanical wave, which means it requires a physical medium—such as air, water, or steel—to travel. Unlike light, which can travel through the vacuum of space, sound cannot exist where there is no matter. It travels through the compression and rarefaction of the medium's particles Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64. Imagine a slinky: when you push one end, a pulse of tightly packed coils (compression) followed by stretched-out coils (rarefaction) moves through it. This is exactly how sound energy moves.

Sound is also classified as a longitudinal wave. In these waves, the individual particles of the medium vibrate back and forth in a direction parallel to the direction the wave is moving. This is different from transverse waves (like ripples in water or S-waves in earthquakes), where particles move up and down, perpendicular to the wave's path Physical Geography by PMF IAS, Earths Interior, p.62. Because sound relies on particles bumping into one another, it is highly sensitive to how closely packed those particles are and how quickly they "spring back" after being pushed.

The speed of sound is determined by the elasticity and density of the medium. Generally, sound travels fastest in solids, then liquids, and slowest in gases. While it might seem counterintuitive that a "denser" material allows for faster travel, in physics, higher density in solids often correlates with higher elasticity (the ability to return to the original shape quickly). This allows the compression and rarefaction cycles to pass through the material much more efficiently Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64.

Wave Property	Mechanical Nature	Longitudinal Nature
Requirement	Must have a material medium (Solid/Liquid/Gas).	Particles must be able to vibrate back and forth.
Mechanism	Cannot travel in a vacuum.	Travels via pressure changes (Compression/Rarefaction).
Seismic Analog	P-waves (Primary waves) Physical Geography by PMF IAS, Earths Interior, p.61.	P-waves move in the direction of propagation.

Sources: Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64; Physical Geography by PMF IAS, Earths Interior, p.61; Physical Geography by PMF IAS, Earths Interior, p.62

2. Characteristics of Sound Waves (basic)

To understand sound, we must first look at the DNA of a wave. A sound wave is a mechanical disturbance that travels through a medium, and its behavior is defined by five key characteristics: Wavelength, Frequency, Amplitude, Time Period, and Speed.

When we visualize a wave, we see alternating high points and low points. The highest point is called the Crest, and the lowest is the Trough Fundamentals of Physical Geography, Class XI NCERT, Movements of Ocean Water, p.109. The physical distance between two consecutive crests is the Wavelength (represented by the Greek letter lambda, λ). Closely related to this is Frequency (f), which is the number of waves passing a fixed point in one second Physical Geography by PMF IAS, Tsunami, p.192. There is an inverse relationship here: if the frequency is high, the wavelength must be short, and vice versa Physical Geography by PMF IAS, Earths Atmosphere, p.279.

Two other vital properties determine how we "experience" sound:

• Amplitude: This is the vertical distance from the center line to the crest. In sound, amplitude determines Loudness. The higher the amplitude, the more energy the wave carries. We measure the intensity of sound in Decibels (dB); notably, an increase of just 10 dB roughly doubles the perceived loudness Environment, Shankar IAS Academy, Environmental Pollution, p.80.
• Speed: This is the rate at which the sound travels. Unlike light, sound speed is heavily dependent on the medium. It travels fastest in Solids (due to high elasticity and rigid structure), slower in Liquids, and slowest in Gases. For instance, sound travels at roughly 343 m/s in air but jumps to over 1400 m/s in water.

Characteristic	Physical Meaning	Perceptual Quality
Frequency	Cycles per second (Hertz)	Pitch (High vs. Low voice)
Amplitude	Maximum displacement	Loudness (Quiet vs. Roar)
Wavelength	Distance between peaks	Spatial extent of the wave

Sources: Fundamentals of Physical Geography, Class XI NCERT, Movements of Ocean Water, p.109; Physical Geography by PMF IAS, Tsunami, p.192; Physical Geography by PMF IAS, Earths Atmosphere, p.279; Environment, Shankar IAS Academy, Environmental Pollution, p.80

3. Mechanism of Wave Propagation in Media (intermediate)

To understand wave propagation, we must first look at waves as a transfer of energy rather than a transfer of matter. When a mechanical wave (like sound or a seismic P-wave) travels through a medium, it does so by causing the particles of that medium to vibrate. In longitudinal waves, this vibration occurs parallel to the direction of travel, creating alternating regions of compression (where particles are squeezed together) and rarefaction (where they are stretched apart) Fundamentals of Physical Geography (NCERT), The Origin and Evolution of the Earth, p.20. This "squeezing and stretching" exerts pressure on neighboring particles, passing the energy forward like a relay race.

The efficiency of this energy transfer depends heavily on how quickly the medium can "spring back" after being disturbed—a property known as elasticity or rigidity. This is why sound travels fastest in solids, followed by liquids, and slowest in gases. In a solid, the atoms are tightly bound by strong intermolecular forces; when one atom moves, its neighbors respond almost instantaneously. In contrast, gas molecules are far apart and must physically collide to pass on the vibration, leading to a much slower propagation speed Physical Geography (PMF IAS), Earth's Interior, p.60.

A common point of confusion in competitive exams is the relationship between density and velocity. While it might seem that a "thicker" or denser material would be harder to move, in the case of sound, higher density in materials often correlates with significantly higher elasticity. For instance, even though steel is much denser than air, its extreme rigidity allows sound to travel through it at approximately 5960 m/s, compared to just ~343 m/s in air Physical Geography (PMF IAS), Earth's Magnetic Field (Geomagnetic Field), p.64. However, if you compare two materials of similar elasticity, the denser one will actually slow the wave down—this is why sound travels faster in Steel than in Copper, as steel has a superior ratio of elasticity to density.

Medium State	Primary Mechanism	Relative Speed
Solid	High elasticity/rigidity; rapid atomic response.	Fastest
Liquid	Moderate molecular bonding; less rigid than solids.	Intermediate
Gas	Random molecular collisions; low elasticity.	Slowest

Sources: Fundamentals of Physical Geography (NCERT), The Origin and Evolution of the Earth, p.20; Physical Geography (PMF IAS), Earth's Interior, p.60; Physical Geography (PMF IAS), Earth's Magnetic Field (Geomagnetic Field), p.64

4. Acoustic Phenomena: Reflection and SONAR (intermediate)

To understand how we map the deep ocean or detect submarines, we must first master the principle of Reflection. Just like light, sound waves obey the Laws of Reflection: the angle of incidence equals the angle of reflection, and the incident wave, the reflected wave, and the 'normal' all lie in the same plane Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.139. In acoustics, we call a distinct reflected sound an echo. For a human to hear a clear echo, the reflecting surface must be far enough away (at least 17.2 meters) so that our brain can distinguish the reflection from the original sound.

The efficiency of this reflection depends on the medium the sound is traveling through. Sound is a mechanical wave; it requires a medium to propagate. Interestingly, sound travels fastest in solids, followed by liquids, and slowest in gases. This is because solids have higher elasticity—the particles are tightly bound and return to their original position much faster after a disturbance, passing the energy along quickly. For example, sound travels at approximately 5960 m/s in steel, roughly 1500 m/s in water, and only about 343 m/s in air.

SONAR (Sound Navigation and Ranging) is a technology that harnesses these reflections to 'see' underwater. It consists of a transmitter and a detector installed on a ship or submarine. The transmitter emits ultrasonic waves (high-frequency sound beyond human hearing). These waves travel through the water, strike an object like the seabed or a school of fish, and reflect back. By measuring the time (t) it takes for the pulse to return and knowing the speed (v) of sound in water, we calculate the distance (d) using the Echo Ranging formula: 2d = v × t. We use '2d' because the sound travels to the object and back.

Medium Type	Typical Speed (m/s)	Key Reason
Solids (e.g., Steel)	~5000 - 6000	Highest elasticity/rigidity
Liquids (e.g., Water)	~1400 - 1500	Moderate elasticity
Gases (e.g., Air)	~330 - 350	Low elasticity and density

Sources: Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.139

5. The Doppler Effect in Sound (intermediate)

The Doppler Effect is an apparent change in the frequency (or pitch) of a wave when there is relative motion between the source of the sound and the observer. It is crucial to understand that the source continues to emit the same frequency; the change is only in how the observer perceives it. Think of a passing ambulance: as it speeds toward you, the siren sounds high-pitched, but as soon as it passes, the pitch drops noticeably to a lower tone.

To understand why this happens, recall that sound is a mechanical wave that travels through the compression and rarefaction of a medium Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64. When a source moves toward an observer, it "catches up" to the sound waves it has already emitted. This bunches the wavefronts closer together, decreasing the wavelength and increasing the frequency (pitch). Conversely, when moving away, the wavefronts are stretched out, increasing the wavelength and decreasing the frequency.

The velocity of sound itself remains constant for a given medium, such as air or water, because it depends on the medium's internal properties like elasticity and density Physical Geography by PMF IAS, Earths Interior, p.60. Therefore, the Doppler Effect doesn't change the speed of the sound waves; it only changes the number of waves hitting your ear per second. This principle is not just a curiosity of acoustics; it is the foundation for technologies like Police RADAR and Medical Ultrasound (to measure blood flow).

Relative Motion	Wavefronts	Perceived Pitch
Moving Toward	Compressed (Shorter λ)	Higher Frequency
Moving Away	Expanded (Longer λ)	Lower Frequency

Sources: Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64; Physical Geography by PMF IAS, Earths Interior, p.60

6. Factors Influencing the Speed of Sound (exam-level)

To understand what makes sound travel faster or slower, we must look at sound for what it fundamentally is: a mechanical wave that moves through the compression and rarefaction of molecules. Unlike light, which can travel through a vacuum, sound requires a medium to "push" against. Therefore, the speed of sound is determined by how quickly the molecules of a medium can pass a vibration to their neighbors and return to their original position.

The most critical factor is the nature of the medium (Solid, Liquid, or Gas). Sound travels fastest in solids, slower in liquids, and slowest in gases. This is primarily due to the medium's elasticity—the ability of a material to maintain its shape and move back into place after a force is applied. While solids are denser than gases, their high Young's Modulus (a measure of stiffness) allows them to transmit mechanical energy far more efficiently. For instance, sound travels at roughly 343 m/s in air, but speeds up to approximately 1,482 m/s in water and reaches nearly 5,960 m/s in steel Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64.

Medium	Approx. Speed (m/s)	Primary Reason
Solids (Steel)	~5,960	High elasticity and rigid molecular structure.
Liquids (Water)	~1,482	Lower elasticity than solids but denser than gases.
Gases (Air)	~343	Highly compressible with weak molecular bonds.

Beyond the state of matter, environmental factors like temperature and humidity play a massive role, especially in our atmosphere:

• Temperature: In gases, the speed of sound is directly proportional to the square root of the absolute temperature. As temperature rises, molecules move faster and collide more often, passing the sound wave along more quickly. This is why sound speed follows the temperature profile of the atmospheric layers Physical Geography by PMF IAS, Earths Atmosphere, p.274.
• Humidity: Surprisingly, sound travels faster in humid air than in dry air. This is because water vapor (H₂O) is less dense than the Nitrogen (N₂) and Oxygen (O₂) it replaces. Lower density in a gas (at constant pressure) translates to higher sound velocity.

Sources: Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64; Physical Geography by PMF IAS, Earths Atmosphere, p.274

7. Speed Comparison across States of Matter (exam-level)

To understand why sound travels at different speeds, we must look at the particulate nature of matter. Sound is a mechanical wave that moves through the vibration of particles. In solids, the constituent particles are closely packed and have very strong interparticle interactions Science Class VIII, Particulate Nature of Matter, p.113. Because the molecules are so close together and tightly bound, they can pass the vibration (energy) to their neighbors almost instantaneously. In contrast, in gases, particles are far apart and move randomly, making the energy transfer much slower.

While it might seem counterintuitive that sound travels faster in denser materials, the speed of sound is actually determined by a tug-of-war between two properties: elasticity (how quickly a material returns to its original shape) and density. Formally, the speed of sound (v) is proportional to the square root of the medium's elasticity (E) divided by its density (ρ), expressed as v ∝ √(E/ρ). Although solids are denser than gases, their elasticity (rigidity) is significantly higher, which more than compensates for the density, resulting in much higher speeds.

Medium State	Representative Material	Approximate Speed (m/s)	Reasoning
Gas	Air (at 20°C)	~343	Particles are far apart; slow energy transfer.
Liquid	Water	~1,480	Particles are closer than in gases but can still move past each other.
Solid (Metal)	Steel	~5,960	Highly rigid structure; particles respond rapidly to disturbances.

Even within the same state, materials differ. For example, metals are known for their sonority—the ability to produce a ringing sound due to their specific structural properties Science Class VII, The World of Metals and Non-metals, p.46. If we compare two metals like Steel and Copper, sound travels faster in Steel because it has a higher Young’s modulus (a measure of stiffness) compared to Copper, despite both being solid conductors.

Sources: Science Class VIII, Particulate Nature of Matter, p.113; Science Class VII, The World of Metals and Non-metals, p.46

8. Solving the Original PYQ (exam-level)

You’ve just mastered the fundamental mechanics of longitudinal waves, specifically how the elasticity and density of a medium dictate the velocity of sound. This question is a classic application of the principle that sound travels fastest in solids, then liquids, and slowest in gases. This occurs because the molecules in solids are more tightly packed and bound by stronger intermolecular forces, allowing vibrational energy to transfer much more rapidly than in the sparse molecular environment of a gas like Air.

To arrive at the correct answer, we must categorize the mediums by their state of matter: Air (1) is a gas, Water (4) is a liquid, while Steel (2) and Copper (3) are both solids. This immediately identifies Air as the slowest and Water as the next level. The real coaching challenge lies in distinguishing between the two metals. While Copper is a dense metal, Steel possesses a significantly higher Young’s Modulus (stiffness/elasticity). Because elasticity plays a more dominant role than density in determining speed, sound travels faster through Steel (~5960 m/s) than Copper (~3750 m/s). Following this logic, the sequence must be 1 < 4 < 3 < 2, which makes (C) the correct choice.

UPSC often uses options like (A) and (D) as traps to test if you can differentiate between specific materials within the same state. A common mistake is to assume that higher density always means lower speed; however, as explained in Physical Geography by PMF IAS, the high bulk modulus of solids more than compensates for their density. Options (B) and (D) are distractors that incorrectly place Water as slower than Air, a fundamental error that ignores the density-elasticity relationship required for mechanical wave propagation. Always prioritize the state of matter first, then refine based on elastic properties.