Which one of the following statements is not correct?

1. Nature of Sound: Mechanical vs Electromagnetic Waves (basic)

To understand the world of acoustics, we must first distinguish between how energy travels. At its core, sound is a mechanical wave. This means it requires a physical medium — whether a solid, liquid, or gas — to propagate. It moves by vibrating the particles of that medium in a series of compressions (bunched up particles) and rarefactions (spread out particles). This is why you cannot hear sound in the vacuum of outer space; there are simply no molecules to pass the vibration along. This behavior is very similar to P-waves (primary waves) produced during earthquakes, which also travel through the Earth's interior by compressing and stretching the material they pass through Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.20.

In contrast, electromagnetic (EM) waves, such as light or radio waves, are fundamentally different. They do not require a medium and can travel effortlessly through a vacuum. While sound is a longitudinal wave (the vibration is parallel to the direction of travel), light is a transverse wave (the vibration is perpendicular to the direction of travel). Interestingly, the medium affects these waves in opposite ways: while an increase in density often facilitates faster sound travel due to increased elasticity, it actually slows down light by increasing the 'refractive index' or effective path length Physical Geography by PMF IAS, Earths Magnetic Field, p.64.

Feature	Mechanical Waves (Sound)	Electromagnetic Waves (Light)
Medium Requirement	Necessary (Solid, Liquid, or Gas)	Not necessary (can travel in vacuum)
Wave Type	Longitudinal (Compression/Rarefaction)	Transverse (Electric/Magnetic Oscillations)
Speed	Relatively slow (~343 m/s in air)	Extremely fast (~300,000,000 m/s)
Effect of Density	Velocity typically increases with density/elasticity	Velocity decreases as density/refractive index increases

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.20; Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64

2. Characteristics of Sound: Frequency, Amplitude, and Pitch (basic)

To understand sound, we must first look at its nature as a mechanical wave. Sound travels through a medium (like air, water, or solids) by creating a series of compressions (high-pressure zones) and rarefactions (low-pressure zones). This movement is facilitated by the elasticity and density of the medium Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64. The two most fundamental physical properties of these waves are Frequency and Amplitude, which our brains interpret as Pitch and Loudness.

Frequency refers to the number of vibrations or cycles a sound wave completes in one second, measured in Hertz (Hz). Our subjective perception of frequency is called Pitch. A high-frequency sound (like a whistle or a bird chirping) has a high pitch, while a low-frequency sound (like a bass drum or a man’s deep voice) has a low pitch. On the other hand, Amplitude measures the maximum displacement of the particles in the medium from their resting position—essentially the "height" of the wave. We perceive amplitude as Loudness or intensity. The greater the energy carried by the wave, the higher its amplitude and the louder the sound.

Characteristic	Physical Property	Subjective Perception	Unit/Measure
Frequency	Number of cycles per second	Pitch (Shrillness vs. Bass)	Hertz (Hz)
Amplitude	Max displacement/Energy	Loudness (Volume)	Decibels (dB)

It is crucial to note that while frequency and amplitude define the quality of the sound, they generally do not affect the speed at which sound travels through a specific medium. The speed of sound is primarily determined by the properties of the medium itself, such as temperature and humidity. For instance, sound travels faster in warmer air because the molecules have more kinetic energy. Similarly, increased humidity slightly increases the speed of sound because water vapor is less dense than dry air, reducing the overall mean molecular weight of the air mixture. Prolonged exposure to high-amplitude (loud) sounds can lead to physiological issues, including hearing loss and increased blood pressure Environment, Shankar IAS Academy, Environmental Pollution, p.81.

Sources: Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64; Environment, Shankar IAS Academy, Environmental Pollution, p.81

3. Propagation in Media: Solids, Liquids, and Gases (intermediate)

To understand how waves move through different materials, we must first distinguish between mechanical waves (like sound) and electromagnetic waves (like light). Sound requires a physical medium to travel because it moves via the compression and rarefaction of particles Physical Geography by PMF IAS, Earth's Magnetic Field (Geomagnetic Field), p.64. Light, however, is a transverse wave that travels fastest in a vacuum and actually slows down when it enters denser transparent media like water or glass Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.148.

For sound, the speed of propagation is determined by two competing factors: Elasticity (how quickly the medium returns to its original shape) and Density. While we often assume higher density slows sound down, in solids, the high "stiffness" or elasticity far outweighs the effect of density. This is why sound travels significantly faster in solids than in liquids, and faster in liquids than in gases. For instance, seismic P-waves can travel at over 13 km/s in the Earth's lower mantle due to extreme pressure making the material highly elastic Physical Geography by PMF IAS, Earth's Interior, p.61.

Medium State	Relative Speed	Primary Driver
Solids	Fastest	High Elasticity (Stiffness) overcomes high density.
Liquids	Intermediate	Higher density than gases, but less elastic than solids.
Gases	Slowest	Low density, but very low elasticity (highly compressible).

In the atmosphere, the speed of sound is sensitive to environmental changes. Temperature is the dominant factor: as air warms, particles move faster, transmitting vibrations more quickly. Interestingly, Humidity also increases the speed of sound. This is because water vapor (H₂O) molecules are lighter than the Nitrogen (N₂) and Oxygen (O₂) molecules they replace. This slightly reduces the average density of the air, allowing sound to propagate faster. Contrary to common intuition, Pressure changes (at a constant temperature) have virtually no effect on the speed of sound in an ideal gas, as the change in density perfectly offsets the change in pressure.

Sources: Physical Geography by PMF IAS, Earth's Magnetic Field (Geomagnetic Field), p.64; Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.148; Physical Geography by PMF IAS, Earth's Interior, p.61

4. Reflection and Absorption: Echo, SONAR, and Ultrasound (intermediate)

When sound waves encounter a boundary between two different media, they undergo reflection and absorption. Reflection is the process where sound 'bounces' off a surface, following the same laws as light: the angle of incidence equals the angle of reflection. If the reflecting surface is distant enough, we hear an echo. For a human to distinguish an echo from the original sound, the reflected sound must reach the ear at least 0.1 seconds after the original sound. Given the average speed of sound in air (approx. 344 m/s), this requires the reflecting surface to be at least 17.2 meters away. Soft, porous materials like carpets or heavy curtains act as absorbers, converting sound energy into heat and reducing unwanted reflections.

While sound in air travels as a longitudinal wave, it is important to distinguish this from other wave types. For instance, in ocean waves, while the wave train moves forward, the actual water molecules move in a circular motion, returning to nearly their original position after the wave passes Physical Geography by PMF IAS, Tsunami, p.192. In the context of sound, we use high-frequency waves called Ultrasound (frequencies above 20,000 Hz) for specialized applications. A primary application is SONAR (Sound Navigation and Ranging), which calculates the distance to underwater objects by measuring the time interval between the transmission of an ultrasonic pulse and the reception of its echo. This principle is vital for mapping the ocean floor and detecting shoals of fish.

The efficiency of these acoustic technologies depends heavily on the medium's properties. In the atmosphere, the speed of sound is primarily determined by temperature. As temperature (T) increases, the kinetic energy of the molecules increases, allowing sound to travel faster (v ∝ √T) Physical Geography by PMF IAS, Earth's Atmosphere, p.274. Interestingly, humidity also plays a role: moist air is actually less dense than dry air because water vapor (H₂O) has a lower molecular weight than Nitrogen (N₂) or Oxygen (O₂). Therefore, an increase in humidity leads to a slight increase in the speed of sound. Importantly, for standard small-amplitude waves, the speed of sound remains independent of its frequency or pressure.

Sources: Physical Geography by PMF IAS, Tsunami, p.192; Physical Geography by PMF IAS, Earth's Atmosphere, p.274

5. Comparative Physics: Sound vs Light Waves (intermediate)

To understand the universe, we must distinguish between the two primary ways energy travels: mechanical waves (sound) and electromagnetic waves (light). At their core, sound is a physical disturbance that requires a medium—like a series of colliding billiard balls—while light is a self-sustaining oscillation of electric and magnetic fields that can travel through the absolute void of space. Because sound relies on the physical properties of matter, it is a longitudinal wave characterized by compressions and rarefactions. In contrast, light is a transverse wave, and as modern quantum theory suggests, it exhibits a dual nature, behaving as both a wave and a stream of particles known as photons Science, class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.134.

The most fascinating contrast lies in how these waves respond to the density of a medium. For sound, a higher density often implies greater elasticity, allowing the medium to "spring back" faster after being compressed; thus, sound generally travels faster in solids than in liquids, and faster in liquids than in gases. For instance, earthquake P-waves (a type of sound wave) travel significantly faster through the Earth's dense mantle than through the crust Physical Geography by PMF IAS, Earths Interior, p.61. Light behaves in the exact opposite manner: an increase in density increases the refractive index, effectively slowing light down because the "path length" it must navigate becomes more cluttered Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64.

In our atmosphere, sound speed is a "fickle" traveler. It is highly sensitive to Temperature—as air gets hotter, molecules move faster and transmit sound energy more quickly. Interestingly, Humidity also speeds up sound; because water vapor molecules (H₂O) are lighter than the Nitrogen (N₂) and Oxygen (O₂) they replace, humid air is less dense and allows sound to propagate faster. However, in standard linear acoustics, the speed of sound remains independent of its frequency (pitch) or amplitude (loudness). We see the ultimate manifestation of this speed gap during a thunderstorm: the discharge of plasma creates both a flash and a shockwave (thunder) simultaneously, but we see the light instantly while the sound lags behind, traveling at roughly 340 m/s Physical Geography by PMF IAS, Thunderstorm, p.349.

Feature	Sound Waves	Light Waves
Nature	Mechanical (Longitudinal)	Electromagnetic (Transverse)
Medium Required?	Yes (Solid, Liquid, Gas)	No (Can travel in Vacuum)
Speed in Denser Medium	Generally Increases	Generally Decreases
Atmospheric Factors	Speeds up with Temp & Humidity	Speeds up as density drops (Vacuum is fastest)

Sources: Science, class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.134; Physical Geography by PMF IAS, Earths Interior, p.61; Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64; Physical Geography by PMF IAS, Thunderstorm, p.349

6. Environmental Factors Influencing Sound Velocity (exam-level)

To understand why sound travels faster on a hot, humid afternoon than on a cold, dry morning, we must look at the physical properties of the medium. Sound is a mechanical longitudinal wave, meaning it relies on the particles of the medium to bump into one another to transfer energy. The speed at which this happens is primarily determined by two factors: the medium's elasticity and its density. While we often assume denser materials slow down waves, it is actually the high elasticity (the ability to snap back into shape) of solids like iron that makes sound travel much faster in them than in liquids like mercury, despite mercury being denser Physical Geography by PMF IAS, Earths Interior, p.61.
In the atmosphere, temperature is the most dominant factor. As temperature rises, the kinetic energy of air molecules increases, allowing them to vibrate and transmit the sound pulse more rapidly. Mathematically, the velocity of sound in an ideal gas is directly proportional to the square root of its absolute temperature (v ∝ √T). This has a practical implication in geography: since temperature generally decreases with height at the normal lapse rate of 6.5°C per 1,000 meters, sound actually travels slower at higher altitudes in the troposphere FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Solar Radiation, Heat Balance and Temperature, p.70.
Another critical factor is humidity. It is a common misconception that humid air is 'heavier' or denser. In reality, water vapor (H₂O) has a lower molecular weight than Nitrogen (N₂) or Oxygen (O₂). When humidity increases, these lighter water molecules displace the heavier gas molecules, effectively decreasing the density of the air. Since sound velocity is inversely proportional to the square root of density (v ∝ 1/√ρ), sound travels faster in humid air than in dry air. Interestingly, atmospheric pressure alone has no effect on sound speed as long as the temperature remains constant; if you increase pressure, the density increases proportionally, and the two effects cancel each other out.
Lastly, in linear acoustic theory (standard sound levels), the speed is independent of the wave's frequency or amplitude. Whether you shout or whisper, or whether the pitch is high or low, the sound waves will reach a listener at the same time.

Factor	Change in Factor	Effect on Sound Speed
Temperature	Increase ↑	Increase ↑
Humidity	Increase ↑	Increase ↑ (due to lower density)
Pressure	Increase ↑	No Change (if T is constant)
Density	Increase ↑	Decrease ↓

Sources: Physical Geography by PMF IAS, Earths Interior, p.61; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Solar Radiation, Heat Balance and Temperature, p.70

7. Solving the Original PYQ (exam-level)

This question brings together the fundamental principles of acoustic speed and gas laws you have just studied. To solve this, you must apply the relationship between the medium's properties (temperature, density, and molecular weight) and the speed of sound. As you learned, the velocity of sound in an ideal gas depends primarily on the square root of absolute temperature. When temperature rises, molecules move faster, facilitating quicker energy transfer. Similarly, sound speed is inversely proportional to the square root of the mean molecular weight of the air. This is the crucial link: moist air contains water vapor molecules (M ≈ 18) which are lighter than Nitrogen (M ≈ 28) or Oxygen (M ≈ 32). Therefore, adding humidity actually lowers the overall density of the air mixture.

As your coach, I want you to look at Option (C) through the lens of molecular physics. Since humid air is less dense than dry air at the same temperature and pressure, the velocity of sound must increase. The statement in (C) claims it "decreased," making it the correct answer because the question asks for the statement that is not correct. Regarding the other options, UPSC often tries to trick candidates with Option (B); however, remember that while pressure affects density, their effects on sound speed cancel each other out as long as temperature remains constant. Furthermore, Option (D) confirms the linear nature of sound waves, where the physical properties of the medium—not the sound's own characteristics like loudness (amplitude) or pitch (frequency)—determine its speed.

The primary trap here is a common misconception about atmospheric density. Many students intuitively feel that "humid" air is "heavier" or thicker, but in the realm of Physical Geography and Physics, water vapor displaces heavier molecules, making the air lighter. Always double-check "NOT" questions carefully, as it is easy to accidentally pick a true statement like (A) or (B) in a rush. As noted in Physical Geography by PMF IAS, these nuances in the structure of the Earth's atmosphere are frequent targets for UPSC examiners seeking to test conceptual depth over rote memorization.