1. (a) Air
2. (b) Wave

1. In a graph plotting density against distance, regions where the curve rises above the average density line indicate compressions. The highest point on this rise is the crest, signifying maximum density.
2. Conversely, sections where the curve falls below the average density line represent rarefactions, with the lowest point (the trough) indicating minimum density.
3. This visual representation clearly shows the periodic fluctuation and spatial distribution of these regions.

1. No, the astronauts cannot talk to each other directly in space or hear the sound of metal clanking.
2. Sound needs a material medium (like air, water, or solids) to travel. In space, there is no atmosphere (vacuum), so sound waves cannot propagate. Therefore, even though they may speak or the metal may collide, the sound cannot reach the other astronaut.

1. Where v = velocity/speed of the wave, ν = frequency of the wave, and λ = wavelength of the wave.

1. Let the time period of a wave be T seconds. In T seconds, the number of waves generated = 1.
2. So, in 1 second, the number of waves generated = 1/T. But the number of waves generated in 1 second is frequency. Therefore, υ = 1/T.
3. Now, Velocity = Distance travelled / Time taken = λ/T = (1/T) × λ = υ × λ.

1. Sound waves are propagated because of compressions and rarefactions in the particles of the medium.
2. The region where particles come closer together is called compression, and the region where particles move farther apart from each other is called rarefaction. As the source vibrates back and forth, it alternately pushes air particles together (compression) and pulls them apart (rarefaction), and this pattern travels outward as a sound wave.

1. (a) Time Period (T): The time required by a wave to complete one vibration is called the time period.
2. (b) Amplitude (A): The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of a wave.

1. A vacuum is a space devoid of matter, meaning it contains no medium.
2. Sound cannot be transmitted through a vacuum because it requires a material medium (like solids, liquids, or gases) to propagate. The absence of particles in a vacuum prevents the vibrations of sound waves from being carried forward.

1. In a longitudinal wave, the particles of the medium oscillate back and forth. This vibration occurs parallel to the direction in which the wave itself is propagating.
2. This consistent motion allows the disturbance to travel through the medium without the individual particles undergoing significant displacement from their mean positions, transmitting energy efficiently.

1. Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar, which is connected to a vacuum pump.
2. If you press the switch, you will be able to hear the bell. Now start the vacuum pump. As the air in the jar is pumped out gradually, the sound becomes fainter, although the same current is passing through the bell.
3. After some time, when less air is left inside the bell jar, you will hear a very feeble sound. If we evacuate the bell jar completely, no sound is heard.

1. Given: λ = 1 cm = 1×10⁻² m, v = 330 m/s.
2. Frequency, υ = 330/(1×10⁻²) = 33,000 Hz.
3. This frequency is more than 20,000 Hz (maximum frequency audible to man). Hence, it cannot be heard by a normal human being.

1. Frequency of the source = 100 Hz means the sound source vibrates 100 times in one second.
2. Therefore, vibrations made in 1 minute (60 sec) = 100 × 60 = 6000 times.

1. Given: Frequency, υ = 2 kHz = 2000 Hz. Wavelength, λ = 35 cm = 0.35 m.
2. Speed, v = λυ = 0.35 × 2000 = 700 m/s.
3. Time taken to travel distance d = 1.5 km = 1500 m: t = d/v = 1500/700 = 15/7 ≈ 2.1 s.

1. (a) False. The change in density (or pressure) from the maximum value to the minimum value and again to the maximum value is called an oscillation, not amplitude.
2. (b) True.
3. (c) True.

1. (a) Quality (Timbre)
2. (b) Increases
3. (c) Wavelength

1. (i) In the graphical figure, both waves are of the same frequency but with different amplitude. In Wave A, amplitude is less, and in Wave B, amplitude is higher.
2. (ii) The relationship between velocity of sound, wavelength, and frequency is: Velocity of sound = frequency × wavelength.
3. (iii) The term is Amplitude.
4. (iv) The unit of frequency is hertz (Hz).

(i) Greater Wavelength

1. Wave (a) has the greater wavelength.
2. Reasoning: Wavelength (λ) is the horizontal distance between two consecutive peaks (crests) or troughs. Looking at the X-axis, the distance between peaks in graph (a) is significantly larger than in graph (b). Since the scales are the same, a longer physical distance represents a greater wavelength.

(ii) Smaller Amplitude

1. Wave (a) has the smaller amplitude.
2. Reasoning: Amplitude is the maximum vertical displacement of the wave from its central equilibrium position (X-axis). The height of the peaks in wave (a) is much shorter compared to wave (b). Therefore, wave (a) represents a softer sound with smaller amplitude, while wave (b) represents a louder sound with greater amplitude.

1. (a) To hear the sound of the coming train because sound travels within the rail track and helps get an idea of the distance of an incoming train.
2. (b) Sound travels faster through copper (solid) than water (liquid), since sound generally travels fastest in solids, then liquids, then gases.
3. (c) For safety, as he cared for his friend and wanted to prevent an accident from an approaching train.

1. A tuning fork is a U-shaped instrument, typically constructed from steel or aluminum, featuring a stem and two parallel sides called prongs or tines.
2. Its primary application in sound experiments involves striking these tines on a resilient surface, causing them to vibrate at a specific frequency.
3. This vibration produces a pure tone, making it invaluable for demonstrating the relationship between vibration and sound, and for precise pitch reference or calibration in various scientific and musical contexts.

1. The fundamental principle of sound production is the vibration of an object. When an object undergoes periodic to-and-fro motion, it displaces the surrounding medium, creating sound waves.
2. In musical instruments, various components vibrate to produce sound. Examples include vibrating strings in guitars, vibrating membranes in drums, and vibrating air columns in flutes or trumpets.
3. Often, multiple vibrating parts interact to create complex sounds, with the primary vibrating object being the source.

1. (i) Dolphins, Whales use echolocation for navigation or hunting.
2. (ii) Ultrasonic waves are used in medical ultrasonography (ultrasound scanning) for imaging internal organs.
3. (iii)(i) Ultrasonic waves → Dog / Bat / Cat (any one). (ii) Infrasonic waves → Elephant / Whale (any one).

OR

1. Humans: Cannot hear it, because the human audible range is approximately 20 Hz to 20,000 Hz, and 25 kHz exceeds this range.
2. Dogs: Can hear it, because dogs can detect ultrasonic frequencies above 20 kHz, including 25 kHz.

1. (i) Wavelength is the distance between two consecutive compressions or two consecutive rarefactions in a sound wave. SI Unit: metre (m).
2. (ii) The speed of sound in air increases with temperature. According to the passage, the speed of sound is about 331 m s⁻¹ at 0°C and 344 m s⁻¹ at 22°C. Thus, a rise in temperature increases the speed of sound.
3. (iii) For a given medium, the speed of sound v remains constant. Using v = λν: If frequency (ν) is doubled, the wavelength (λ) must become half to keep v constant. Also, T = 1/ν, so when frequency is doubled, time period becomes half.

OR

1. Given: v = 340 m s⁻¹, ν = 680 Hz.
2. Wavelength: λ = v/ν = 340/680 = 0.5 m.
3. Time Period: T = 1/ν = 1/680 ≈ 0.00147 s (≈ 1.47 × 10⁻³ s).

1. (i) The main cause of sound production is vibration. When an object vibrates, it creates disturbances in the surrounding medium, which travel as sound waves and are heard as sound.
2. (ii) A source of sound is any vibrating object that produces sound. Examples include musical instruments, vibrating strings, or objects like tuning forks that generate sound through motion.
3. (iii) Musical instruments produce sound through vibrations of strings, membranes, or air columns. These vibrations create sound waves, and in many instruments, multiple vibrating parts combine to produce complex and rich sounds.

OR

1. A tuning fork produces sound when its prongs are struck, causing them to vibrate. These vibrations disturb the surrounding air, creating sound waves that travel to our ears and are heard as sound.

1. (i) The number of waves produced per second is called the frequency of the wave.
2. (ii) Sound waves audible to an average human ear range from 20 Hz to 20 kHz.

(iii) Calculations

1. a. Wavelength: The distance between a crest and next trough is λ/2. Given λ/2 = 50 cm, so λ = 100 cm or 1 m.
2. b. Frequency: One vibration (oscillation) consists of one trough and one crest. In 0.2 s, number of vibrations = 20. So, in 1 s, number of vibrations = 20/0.2 = 100. Frequency = 100 Hz.
3. c. Time period: T = 1/frequency = 1/100 = 0.01 sec.

1. A curve showing density (or pressure) versus distance is a sine-shaped wave, where the crests represent regions of compression (maximum density) and the troughs represent regions of rarefaction (minimum density).

Wavelength

1. Wavelength is the distance between two consecutive compressions or two consecutive rarefactions of a sound wave. Wavelength is represented by the Greek letter λ (lambda). The SI unit of wavelength is metre (m).

Time period

1. Time period is the time taken to travel the distance between any two consecutive compression or rarefaction from a fixed point. The time period of a sound wave is represented by the letter 'T'. The SI unit of time period is second (s).

(i) Same amplitude, different frequency

Two waves with same amplitude but different frequency

(ii) Same frequency, different amplitude

Two waves with same frequency but different amplitude

(iii) Different amplitudes and different wavelengths

Two waves with different amplitudes and different wavelengths

1. (i) Timbre or quality of sound, which is distinct from its pitch and intensity, helps to identify your friend by his voice while sitting with others in a dark room.

(ii) Relationship and Calculation

1. Given: λ = 4 cm = 0.04 m, v = 20 m/s. Find frequency (ν) and time period (T).
2. ν = v/λ = 20/0.04 = 500 Hz.
3. T = 1/ν = 1/500 = 0.002 s.

1. (i) Sound is produced when a material body is made to vibrate with some mechanical energy. So, the school bell is struck with a hammer to make it vibrate, and thus the sound is produced.

(ii) Depth calculation

1. Time taken by the signal to reach the bottom and come back = 4.8 s. Speed of sound in water = 1500 m/s.
2. Depth of ocean, h = (v × t)/2 = (1500 × 4.8)/2 = 1500 × 2.4.
3. h = 3600 m.