1. Setup: An electric bell is placed inside a sealed glass bell jar. The bell jar is connected to a vacuum pump. The bell is switched on and its sound is clearly heard through the glass jar.
2. Pumping out air: As the vacuum pump slowly removes the air from inside the bell jar, the sound of the ringing bell becomes gradually fainter and fainter, even though the bell can still be seen ringing.
3. Near-vacuum state: When most of the air has been removed (near-vacuum is created), almost no sound can be heard, even though the bell continues to ring visibly.
4. Restoring air: When air is allowed back into the bell jar, the sound gradually returns to its original loudness.
5. Conclusion: This experiment shows that sound cannot propagate in vacuum. Sound needs a material medium (solid, liquid, or gas) to travel. Such waves are called mechanical waves.
6. Application to outer space: Outer space is a near-vacuum. Therefore, astronauts doing spacewalks cannot hear each other speak or hear sounds like metal objects clanking. They must use special electronic communication devices fitted in their spacesuits.

1. Average state (piston at rest): The air inside the tube has a uniform, average density. The particles are distributed evenly.
2. Piston moves forward: The piston pushes the nearby air particles forward, compressing them. This creates a region of higher density called a compression (C). This compression then travels forward as the compressed particles collide with the next set of particles.
3. Piston moves backward: The piston pulls back, and the air near the piston becomes less dense. This creates a region of lower density called a rarefaction (R). This rarefaction also travels forward through collisions between particles.
4. Oscillating piston: As the piston oscillates repeatedly, it creates a series of alternating compressions and rarefactions that travel through the air away from the source.
5. Key note: The particles of the medium do not travel with the wave — they only oscillate about their mean positions. Only the disturbance (the density variation) travels forward.
6. Definitions:
   • Compression: A region where air density is higher than average.
   • Rarefaction: A region where air density is lower than average.
   • Sound wave: The disturbance consisting of a series of alternating compressions and rarefactions propagating through a medium.

Conclusion: The audible range of wavelengths in air spans from 1.72 cm (for high-pitched sounds) to 17.2 m (for very low-pitched sounds). This shows that lower frequency sounds have much longer wavelengths, and higher frequency sounds have shorter wavelengths. The speed of sound in a given medium remains constant — it is only the wavelength that changes when frequency changes.

Explanation — Why lightning is seen before thunder is heard:

1. Lightning (a flash of light) and thunder (a sound) are produced at the same time and place during a storm.
2. Light travels at an extremely high speed of approximately 3 × 10⁸ m s⁻¹. Over a distance of 2 km, light takes less than 0.000007 seconds — practically instantaneous.
3. Sound, however, travels much more slowly, at about 340 m s⁻¹. Over 2 km, it takes about 6 seconds to reach us.
4. Because light arrives almost instantly while sound takes several seconds, we see the lightning first and then hear the thunder seconds later.
5. The further away the lightning, the greater the time gap between seeing the flash and hearing the thunder.

1. Reflection of sound: Sound waves bounce off obstacles such as solids or liquids. This bouncing back of sound is called the reflection of sound. Sound follows the same laws of reflection as light — the angle of incidence equals the angle of reflection, with all three (incident direction, reflected direction, and normal) lying in the same plane.
2. Echo: When sound reflects off a hard, smooth, and distant surface (such as a mountain, cliff, or a long corridor), and the reflected sound reaches the listener at least 0.1 second after the original sound, it is heard as a separate sound called an echo. Example: Shouting near a mountain or in a long empty corridor. The minimum distance of the reflecting surface must be 17 m (at speed = 340 m s⁻¹).
3. Reverberation: In a large hall or auditorium, sound can undergo multiple reflections from the walls, ceiling, and floor. If the reflected sounds reach the listener with a time gap of less than 0.05 s, the brain cannot distinguish between them. The sound then appears to persist even after the source stops — this is called reverberation. Example: Singing in a large bathroom or speaking in an empty hall causes a prolonged “ringing” effect.
4. Controlling reverberation in auditoriums: Excessive reverberation causes garbled or unclear sound. To control it, auditoriums use: sound-absorbing panels on walls and ceilings, upholstered chairs, heavy curtains, carpets, and other soft, porous materials that absorb rather than reflect sound.

Principle of SONAR:

1. SONAR stands for Sound Navigation And Ranging. It uses the principle of reflection of sound (echolocation).
2. A device called a transducer on a ship emits ultrasonic waves (high frequency, above 20 kHz) into the water.
3. These waves travel through the water and strike underwater objects (submarines, shipwrecks, ocean floor, schools of fish) and reflect back to the ship.
4. The ship receives the reflected waves (echoes) and the time taken for the echo to return is measured.
5. Using the formula distance = (v × t) ÷ 2, the distance, direction, and speed of the underwater object are calculated.

1. Echolocation: The ability to locate objects by emitting sound waves and detecting the reflected echoes is called echolocation. It is essentially a natural form of SONAR used by animals.
2. How bats use echolocation: Bats are nocturnal creatures (active at night) and hunt in complete darkness. Their eyes are not well-suited to navigate in the dark.
3. Bats emit short bursts of ultrasonic waves (with frequencies well above 20 kHz) from their mouths or noses as they fly.
4. These ultrasonic waves travel outward and reflect off nearby objects — including obstacles, trees, walls, and prey like insects.
5. The reflected echoes return to the bat’s large, sensitive ears. The bat’s brain analyses these echoes to determine the position, distance, size, and movement of objects around it.
6. This allows bats to avoid collisions and accurately capture prey even in complete darkness, with remarkable precision.

Other animals that use echolocation: Dolphins and whales use echolocation for navigation and hunting underwater. Some birds also use a form of echolocation.

1. Pitch: Pitch is how frequency is perceived by humans. A sound with a high frequency is perceived as having a high pitch (shrill sounds like a whistle or a siren). A sound with a low frequency is perceived as having a low pitch (deep sounds like thunder or an aircraft rumble). The higher the frequency, the higher the pitch.
2. Loudness: Loudness is how amplitude is perceived by humans. Sounds with larger amplitude are perceived as louder; sounds with smaller amplitude are perceived as softer. Loudness depends not just on amplitude but also on the hearing ability of the listener. It decreases as we move farther from the source because sound spreads over a larger area.
3. Intensity: Intensity is a measurable physical quantity — it is the amount of sound energy passing through a unit area perpendicular to the direction of propagation per unit time. It is not the same as loudness. Loudness is the subjective perception, while intensity is objective and can be measured (in decibels, dB).
4. Decibel scale: Normal conversation ≈ 60 dB. Firecrackers can exceed 100 dB. Rustling leaves are around a few dB. Prolonged exposure to high dB sounds can cause permanent hearing loss.

Verification: We can verify: v = λ × ν = 50 × 100 = 5000 m s⁻¹ ✓

• Medical imaging (Ultrasonography): Used for imaging internal organs such as the kidney, liver, and foetus without surgery.
• Breaking kidney stones: High-intensity ultrasonic waves break kidney stones into smaller pieces that can then be passed out of the body naturally.
• Industrial cleaning: Used for cleaning delicate machine parts, jewellery, and electronic components in industries.
• Detecting defects in metals: Ultrasonic waves are used to detect cracks or defects inside metal blocks during construction and industrial testing (non-destructive testing).
• SONAR: Used for detecting and locating underwater objects (submarines, shipwrecks, ocean floor depth).

Verification of parking sensor: