Light & Sound

🎯 Learning Objective

Learn reflection & refraction, the uses of mirrors and lenses, the speeds of light and sound, and why sound needs a medium.

💡 Concept

• Light travels in straight lines; its speed in vacuum ≈ 3 × 10⁸ m/s (about 3 lakh km per second)
• Reflection = light bouncing back off a surface (mirrors); Refraction = light bending as it passes between media (a straw looks bent in water)
• Plane mirror gives a virtual, erect, same-size image
• Concave mirror converges light — used in torches, vehicle headlights, shaving mirrors, solar cookers and dentists' mirrors
• Convex mirror diverges light and gives a wide view — used as vehicle rear-view mirrors
• Convex lens (converging) is used in a magnifying glass and to correct hypermetropia (far-sightedness)
• Concave lens (diverging) is used to correct myopia (short-sightedness)
• Speed of sound in air ≈ 343 m/s — far slower than light, so we see lightning before we hear thunder
• Sound needs a material medium (solid/liquid/gas) and cannot travel through a vacuum; it moves fastest in solids, slowest in gases
• An echo is reflected sound; a distinct echo needs the reflecting surface to be at least ~17 metres away. Human hearing range = 20 Hz to 20,000 Hz

🧮 Key Formulas

Speed of light (vacuum) ≈ 3 × 10⁸ m/s

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Speed of sound in air ≈ 343 m/s

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Distance for a clear echo ≥ 17 m

✏️ Easy Example

Q. Which mirror is used as a vehicle's rear-view mirror? (a) Plane (b) Concave (c) Convex (d) Cylindrical

1. A rear-view mirror must show a WIDE area behind
2. A convex mirror diverges light and gives a wider field of view

Answer: (c) Convex

🇮🇳 Real-Life Example

During a thunderstorm you always SEE lightning first and HEAR thunder a few seconds later — because light reaches you almost instantly while sound crawls at ~343 m/s. Count the gap and you can estimate how far the storm is.

📝 Exam-Level Example

Q. Sound cannot travel through: (a) water (b) iron (c) air (d) vacuum

1. Sound needs particles of a medium to pass energy along
2. A vacuum has no particles, so sound cannot travel through it

Answer: (d) vacuum

📝 Exam-Level Example

Q. A concave mirror is used in a solar cooker because it:

1. A solar cooker must concentrate sunlight to make heat
2. A concave mirror converges (focuses) the rays to a point

Answer: converges/focuses sunlight

🪄 Memory Trick

ConCAVE = 'caves in' = collects/converges light (torch, cooker). ConVEX = bulges out = spreads/diverges (rear-view). Cave collects, Vex spreads.

⚠️ Common Mistakes

• ❌ Swapping concave (converging) and convex (diverging) mirrors
• ❌ Thinking sound can travel in vacuum (it cannot; light can)
• ❌ Confusing myopia (concave lens) with hypermetropia (convex lens)

🏆 Exam Tips

• ✅ Convex mirror + concave lens both 'spread out' — group them for rear-views and myopia
• ✅ Remember 3 × 10⁸ m/s for light and ~343 m/s for sound — a very common compare question

📌 Summary

• Reflection = bounce back; Refraction = bending between media
• Concave mirror converges (torch, cooker); convex mirror diverges (rear-view)
• Convex lens for far-sight (hypermetropia); concave lens for short-sight (myopia)
• Light ≈ 3 × 10⁸ m/s; sound ≈ 343 m/s and needs a medium