Ch 12 treats sound as a wave phenomenon. Students learn about longitudinal waves, compressions and rarefactions, characteristics of sound, speed calculation, echo, SONAR, and the human ear.
Sound is a longitudinal wave: particles vibrate parallel to the direction of travel, creating compressions (high pressure) and rarefactions (low pressure). Needs a medium to travel (cannot travel through vacuum). Speed depends on medium: v(solid) > v(liquid) > v(gas). Speed in air ≈ 340 m/s at 20°C.
Frequency (f): number of vibrations per second (Hz). Wavelength (λ): distance between successive compressions. Speed v = f × λ. Amplitude: loudness (larger amplitude → louder). Pitch: determined by frequency (higher frequency → shriller). Quality/timbre: distinguishes same note on different instruments.
Echo: reflected sound heard separately (needs minimum distance of 17.2 m from reflecting surface for 340 m/s speed). Ultrasound (>20 kHz): used in medical imaging (sonography), SONAR (measuring ocean depth/detecting submarines), and cleaning. Human ear: pinna collects → eardrum vibrates → ossicles amplify → cochlea converts to electrical signals → brain.
Download: https://ncert.nic.in/textbook/pdf/iesc112.pdf | Complete book: https://ncert.nic.in/textbook/pdf/iesc1ps.zip
Light travels at 3 × 10⁸ m/s (almost instantly), while sound travels at only ~340 m/s. So light from lightning reaches us almost immediately, but sound (thunder) takes longer. The delay tells us how far the lightning is: 3 seconds delay ≈ 1 km away.
Book a Trial + Diagnostic session. Get a personalized Learning Path with clear milestones, tutor match, and a plan recommendation — all within 24 hours.
Book Trial + Diagnostic →