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Heat Light and Sound

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Overview

Heat, light and sound are forms of energy with distinct behaviours: heat flows by conduction, convection and radiation; light reflects and refracts; sound needs a medium.

Key points

Heat moves by conduction (solids), convection (fluids) and radiation (no medium).
Kelvin = Celsius + 273.
Light obeys the laws of reflection; refraction bends light between media.
Convex lenses converge light; concave lenses diverge it.
Sound needs a medium and travels fastest in solids.
Pitch depends on frequency; loudness on amplitude.

Explanation

Echoes need a reflecting surface at least about 17 m away.
Dispersion splits white light into colours.
Black surfaces absorb heat better.
Ultrasound is above 20 kHz.
Sea breezes are caused by convection.

Step by step

Identify the concept or reaction.
Recall the definition, equation or example.
Check balance, units and direction.

Examples

A metal spoon in hot tea gets hot → conduction.
A pencil looks bent in water → refraction.
A thin wire vibrating faster gives a higher note → frequency and pitch.

Traps and advanced points

Trap: Saying sound travels in vacuum.
Trap: Mixing pitch and loudness.
Trap: Adding 32 for kelvin.

Quick revision

Three modes of heat transfer.
Reflection, refraction, lenses.
Pitch ↔ frequency; loudness ↔ amplitude.

Practice MCQs

Sound cannot travel through —
1. air
2. water
3. iron
4. vacuum
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Answer: (d) vacuum. Sound needs a medium.
Heat transfer through solids mainly occurs by —
1. radiation
2. evaporation
3. conduction
4. convection
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Answer: (c) conduction. Particles pass energy to neighbours.
Heat from the Sun reaches the Earth by —
1. reflection
2. conduction
3. convection
4. radiation
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Answer: (d) radiation. Radiation needs no medium.
Sea breezes are caused by —
1. radiation
2. refraction
3. conduction
4. convection
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Answer: (d) convection. Warm air over land rises; cooler sea air moves in.
25 °C in kelvin is about —
1. 273 K
2. 325 K
3. 248 K
4. 298 K
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Answer: (d) 298 K. 25 + 273 = 298.
A concave lens —
1. diverges light
2. reflects all light
3. absorbs all light
4. converges light
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Answer: (a) diverges light. Concave lenses spread light out.
An echo needs a reflecting surface at least about —
1. 1.7 m away
2. 17 m away
3. 170 m away
4. 1 km away
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Answer: (b) 17 m away. About 17 m at normal sound speed.
Black surfaces —
1. only conduct sound
2. absorb heat better
3. reflect all heat
4. do not absorb heat
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Answer: (b) absorb heat better. Dark surfaces are good absorbers and emitters.
A thin wire vibrating faster produces a —
1. no sound
2. higher pitch
3. lower pitch
4. louder sound only
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Answer: (b) higher pitch. Higher frequency → higher pitch.
Splitting of white light into seven colours is called —
1. reflection
2. dispersion
3. refraction only
4. interference
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Answer: (b) dispersion. Different colours bend by different amounts.