Question 13
Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier, and thunder is heard later. Can you explain why?
Thunder is heard a few seconds after lightning is seen because sound travels much slower than light.
Light takes almost negligible time in comparison to sound in reaching us from the place of thunder because speed of light is much more (3 x 108 ms-1) than the speed of sound (= 330 ms-1).
Chapter Overview: Sound
This chapter covers how sound is produced by vibrating objects, how it travels through media (solid > liquid > gas), and its characteristics: frequency (pitch), amplitude (loudness), and time period. The audible range, ultrasound applications, human ear anatomy, and noise pollution are important exam topics.
Board Exam Weightage: 5-7 marks | Difficulty: Moderate to High
Key Formulas & Concepts
| Concept | Details |
|---|---|
| Frequency | f = number of oscillations per second; Unit: Hertz (Hz) |
| Time Period | T = 1/f; Time for one complete oscillation; Unit: seconds |
| Audible Range | 20 Hz to 20,000 Hz (for humans) |
| Sound Speed | Solid (~5000 m/s) > Liquid (~1500 m/s) > Gas (~340 m/s in air at 20°C) |
Must-Know Concepts
- Sound is produced by vibrating objects and needs a medium to travel (cannot travel in vacuum)
- Loudness depends on amplitude; pitch depends on frequency
- Men have deeper voices because their vocal cords are longer (lower frequency)
- Ultrasound (>20,000 Hz): used in medical imaging (ultrasonography), SONAR, cleaning
- Infrasound (<20 Hz): produced by earthquakes, elephants, whales
- Noise pollution causes hearing loss, stress, high blood pressure; controlled by silencers, green belts, ear protection
Loudness vs Pitch
| Property | Loudness | Pitch |
|---|---|---|
| Depends on | Amplitude | Frequency |
| Greater → | Louder sound | Higher (shriller) sound |
| Unit | Decibel (dB) | Hertz (Hz) |
| Example | Shouting vs whispering | Woman's voice vs man's voice |
Common Mistakes to Avoid
- Confusing loudness (amplitude) with pitch (frequency)
- Thinking sound can travel through vacuum
- Errors in T = 1/f calculations (they are inversely related)
- Mixing up ultrasound (>20,000 Hz) and infrasound (<20 Hz)
Scoring Tips
- Practice T = 1/f problems: if f = 500 Hz, T = 0.002 s
- Draw and label the human ear with all parts
- Learn 3 applications each of ultrasound and noise prevention
- Remember: solid > liquid > gas for speed of sound
Frequently Asked Questions
Q: How many marks does this chapter carry in the exam?
A: Approximately 5-7 marks in the annual exam.
Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).
Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.