Chapter 8 · Section 3Textbook page 164

Speed of Sound, Thunder and Lightning ശബ്ദവേഗം

In a monsoon storm you see the lightning first and hear the thunder a few seconds later – yet both happen at the same instant. On this page you race sound through aluminium, steel, water, helium and air, work out how far away a firecracker burst in exam layout, and use your own counting to find how far away a thunderstorm is.

📖 What you need (page 4 of this guide)

speed = distance ÷ time, so distance = speed × time and time = distance ÷ speed. Units: m, s, m/s.
Light travels at about 3 × 10⁸ m/s (300 000 000 m/s); sound in air at only about 350 m/s.
1 km = 1000 m.

1. Why do we hear thunder after the lightning?

ഇടിയും മിന്നലും ഒരേ സമയത്താണ് ഉണ്ടാകുന്നതെങ്കിലും മിന്നൽ കണ്ട ശേഷമാണല്ലോ ഇടിനാദം കേൾക്കുന്നത്. അതെന്തുകൊണ്ടായിരിക്കും?Thunder and lightning happen at the same time, but we hear the thunder only after we see the lightning. Why could that be?

Keep this question in mind – the table below holds the clue, and you will answer it fully in section 5.

2. Speed of sound in different media (Table 8.3)

പട്ടിക 8.3 വിശകലനം ചെയ്ത് ചോദ്യങ്ങളുടെ ഉത്തരം എഴുതൂ. – Analyse Table 8.3 and answer the questions.

State of matter പദാർഥത്തിന്റെ അവസ്ഥ	Medium മാധ്യമം	Speed of sound (m/s) at 25 °C ശബ്ദവേഗം
Solid ഖരം	Aluminium അലുമിനിയം	6423
Solid ഖരം	Steel സ്റ്റീൽ	5944
Liquid ദ്രാവകം	Sea water കടൽജലം	1525
Liquid ദ്രാവകം	Pure water ശുദ്ധജലം	1485
Gas വാതകം	Helium ഹീലിയം	968
Gas വാതകം	Air വായു	347

📊 Table 8.3 as a bar chart

Light (3 × 10⁸ m/s) would need a bar about 47 000 times longer than the aluminium bar – it cannot be drawn on this scale.

Questions (page 164).

എല്ലാ മാധ്യമങ്ങളിലൂടെയും ഒരേ വേഗത്തിലാണോ ശബ്ദം സഞ്ചരിക്കുന്നത്? Does sound travel at the same speed through all media?
പദാർഥം ഏത് അവസ്ഥയിലായിരിക്കുമ്പോഴാണ് അതിലൂടെയുള്ള ശബ്ദവേഗം കൂടുതൽ? കുറവോ? In which state of matter is the speed of sound highest? Lowest?

1. No. The speed is different in different media – from 347 m/s in air to 6423 m/s in aluminium.
2. Highest in solids (aluminium 6423 m/s, steel 5944 m/s), lowest in gases (air 347 m/s); liquids come in between (about 1500 m/s). Why (going further): in a solid the particles are tightly linked to their neighbours, so each push is passed on very quickly; in a gas the particles are far apart and only pass the push on when they bump into each other.

🏁 Virtual lab: the medium race (Table 8.3 + light)

distance m

Medium	Speed (m/s)	Time to cover the distance = distance ÷ speed

The race is shown in slow motion so that even the air pulse finishes in a few seconds; the clock shows the real time. Light needs only a few millionths of a second (µs) – it “arrives” as soon as the race starts.

3. Speed and temperature

⭐ Key idea (textbook page 164)

വ്യത്യസ്ത മാധ്യമങ്ങളിൽ ശബ്ദവേഗം വ്യത്യസ്തമാണ്. മാധ്യമങ്ങളുടെ താപനില വ്യത്യാസപ്പെടുമ്പോൾ അവയിലൂടെയുള്ള ശബ്ദവേഗത്തിനും വ്യത്യാസം ഉണ്ടാകുന്നു. താപനില കൂടുമ്പോൾ ശബ്ദവേഗം കൂടുന്നു.

The speed of sound (ശബ്ദവേഗം) is different in different media. It also changes with the temperature (താപനില) of the medium: when the temperature increases, the speed of sound increases.

🌡 Going further: speed of sound in air at different temperatures

air temperature °C

A handy approximation for air: speed ≈ 331 + 0.6 × (temperature in °C) m/s – about 0.6 m/s faster for every 1 °C. (Not in the textbook – for interest only; values are approximate.)

4. The firecracker at night (page 164)

Question. രാത്രിയിൽ പടക്കം പൊട്ടുമ്പോഴുണ്ടായ വെളിച്ചം കണ്ട് 3 s നു ശേഷം അതിന്റെ ശബ്ദം കേട്ടു എങ്കിൽ സ്ഫോടനം നടന്നത് എത്ര അകലെയായിരിക്കും? (ശബ്ദവേഗം 350 m/s എന്ന് പരിഗണിക്കുക). – At night, the sound of a firecracker was heard 3 s after its flash was seen. How far away did the explosion happen? (Take the speed of sound as 350 m/s.)

The light reaches you almost at once, so the 3 s is the time the sound took. Use distance = speed × time.

🎬 Solution in exam layout – how far away was the firecracker?

Answer: distance = speed × time = 350 m/s × 3 s = 1050 m (about 1 km). The light covers this distance in only 1050 ÷ (3 × 10⁸) = 3.5 × 10⁻⁶ s (3.5 millionths of a second) – practically instantly. More: page 10 ▶

വായുവിലെ പ്രകാശവേഗം വളരെ കൂടുതലാണ് (3 × 10⁸ m/s). അതിനാൽ പടക്കം പൊട്ടുന്ന സന്ദർഭത്തിൽ തന്നെ അതിന്റെ വെളിച്ചം അകലെ നിൽക്കുന്ന ആൾക്ക് കാണുവാൻ കഴിയും. എന്നാൽ വായുവിലെ ശബ്ദവേഗം ഏകദേശം 350 m/s മാത്രമാണ്. – The speed of light in air is very high (3 × 10⁸ m/s), so a person far away sees the flash at the very moment the cracker bursts. But the speed of sound in air is only about 350 m/s.

distance travelled by sound = speed of sound × time = 350 m/s × 3 s = 1050 m

🌴 Kerala connection – the Pooram fireworks

At the Thrissur Pooram fireworks (vedikettu) or Vishu night, people far from the launch site see each flash light up the sky and hear the boom a moment later. Count the seconds: a gap of 2 s means the shell burst about 350 × 2 = 700 m away. People standing right next to the launch site see and hear it together.

⚠ Firecrackers are dangerous: a cracker bursting close by can be louder than about 120 dB (approx.) and can damage hearing and burn skin and eyes. Watch fireworks from far behind the barriers, never light crackers yourself without an adult, and never hold one in your hand.

5. Thunder, lightning and the thunder timer

Q (page 164). പല സന്ദർഭങ്ങളിലും മിന്നൽ കണ്ട് അല്പസമയം കഴിഞ്ഞ് മാത്രം ഇടിനാദം കേൾക്കുന്നതിന്റെ കാരണം വിശദമാക്കുക. – Explain why we often hear thunder some time after seeing the lightning.

Lightning and thunder are produced at the same moment. But light travels at about 3 × 10⁸ m/s, so the flash reaches us almost instantly, while sound travels in air at only about 350 m/s, so the thunder takes several seconds to cover the same distance – roughly 3 s for every kilometre. That is why we see the flash first and hear the thunder later. Why it matters: the longer the gap, the farther away the lightning.

⚡ Virtual lab: the thunder timer

seconds counted between flash and thunder s

Count “one-thousand-one, one-thousand-two…” from the flash until you hear the thunder. Distance ≈ 350 × (seconds) metres, or about 1 km for every 3 s. The animation is speeded up; the clock shows the real seconds.

⚠️ Monsoon safety – lightning

If you can hear thunder, you are close enough to be struck by lightning. Go indoors (or into a closed car or bus) at once and stay there until about 30 minutes after the last thunder. Keep away from open fields, open terraces, tall lone trees, coconut palms, ponds and rivers; do not use wired phones or stand near metal poles during the storm.

🌐 Explore online needs internet · opens in a new tab

📄 The Speed of Sound The Physics Classroom
Why sound is faster in solids than in liquids and gases, and faster in warm air than in cold air.
📄 Measuring the speed of sound Science Learning Hub (NZ)
Time an echo from a distant building and use distance and time to calculate the speed of sound.
📄 Learning Lesson: Determining distance to a Thunderstorm NOAA JetStream
Count the seconds between the flash and the thunder, then use the speed of sound to find how far away the storm is.
📄 Understanding Lightning: Thunder US National Weather Service
Why we see lightning before we hear thunder, and why light is so much faster than sound.

6. Check yourself

🧭 Summary

Speed of sound depends on the medium: fastest in solids (aluminium 6423 m/s, steel 5944 m/s), then liquids (sea water 1525, pure water 1485), slowest in gases (helium 968, air 347 m/s) – Table 8.3, 25 °C.
When the temperature of the medium increases, the speed of sound increases.
Light (3 × 10⁸ m/s) is about a million times faster than sound in air, so we see a flash before we hear the bang.
distance = speed × time: firecracker heard 3 s after the flash → 350 × 3 = 1050 m away.

Next: How do we hear? – The ear and hearing ▶