Heat, Light and Sound
Overview
Heat, Light and Sound form the core of classical physics tested in CG TET Paper II. These topics appear consistently because they connect directly to everyday phenomena that upper-primary students encounter—cooking food, seeing objects, hearing sounds. For the exam, you must understand the mechanisms of energy transfer and wave behaviour, not just memorise definitions.
Questions typically test three areas: modes of heat transfer with real-life examples, laws of reflection/refraction with ray diagrams, and characteristics of sound waves. Expect 3–5 questions from this combined topic. Mastery here also strengthens your pedagogy answers, as examiners value teachers who can explain these concepts through simple experiments.
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Key Concepts
- **Heat is energy in transit** due to temperature difference; it flows from hotter to colder bodies until thermal equilibrium is reached.
- **Three modes of heat transfer**: Conduction (through solids without particle movement), Convection (through fluids with particle movement), Radiation (through electromagnetic waves, no medium needed).
- **Light is an electromagnetic wave** that travels in straight lines (rectilinear propagation) at 3 × 10⁸ m/s in vacuum.
- **Reflection**: Light bounces off a surface; angle of incidence equals angle of reflection (measured from the normal).
- **Refraction**: Light bends when passing from one medium to another due to change in speed; bending is toward normal when entering denser medium.
- **Sound is a mechanical longitudinal wave** requiring a medium; it cannot travel through vacuum.
- **Characteristics of sound**: Pitch (frequency), Loudness (amplitude), Quality/Timbre (waveform).
- **Speed of sound** is fastest in solids, slower in liquids, slowest in gases (opposite to light which travels fastest in vacuum/air).
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Formulas / Key Facts
| Concept | Formula / Fact | |---------|----------------| | Heat absorbed/released | Q = m × c × ΔT (Q = heat, m = mass, c = specific heat, ΔT = temperature change) | | Law of Reflection | Angle of incidence (i) = Angle of reflection (r) | | Snell's Law (Refraction) | n₁ sin i = n₂ sin r OR n = sin i / sin r (for air to medium) | | Refractive index | n = Speed in vacuum / Speed in medium | | Speed of sound in air | Approximately 340 m/s at 20°C | | Wave equation | v = f × λ (velocity = frequency × wavelength) | | Echo condition | Minimum distance to reflector = 17 m (sound needs 0.1 s to be heard distinctly) | | Audible frequency range | 20 Hz to 20,000 Hz for humans | | Infrasound | Below 20 Hz (elephants, earthquakes) | | Ultrasound | Above 20,000 Hz (bats, SONAR, medical imaging) |
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Worked Examples
### Example 1: Heat Calculation **Problem**: How much heat is needed to raise the temperature of 2 kg of water from 25°C to 75°C? (Specific heat of water = 4200 J/kg°C)
**Solution**:
- Given: m = 2 kg, c = 4200 J/kg°C, ΔT = 75 – 25 = 50°C
- Apply formula: Q = m × c × ΔT
- Q = 2 × 4200 × 50 = 420000 J = 420 kJ
**Answer**: 420 kJ of heat is required.
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### Example 2: Reflection **Problem**: A ray of light strikes a plane mirror at 35° to the mirror surface. Find the angle of reflection.
**Solution**:
- Angle to surface = 35°
- Angle of incidence = 90° – 35° = 55° (measured from normal)
- By law of reflection: Angle of reflection = Angle of incidence = 55°
**Answer**: 55° from the normal.
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### Example 3: Sound and Echo **Problem**: A person claps near a cliff and hears the echo after 0.4 seconds. How far is the cliff? (Speed of sound = 340 m/s)
**Solution**:
- Total distance travelled by sound = speed × time = 340 × 0.4 = 136 m
- This is the round-trip distance (to cliff and back)
- Distance to cliff = 136 / 2 = 68 m
**Answer**: The cliff is 68 m away.
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Common Mistakes
| Wrong Thinking | Correct Fix | |----------------|-------------| | "Radiation needs a medium like conduction" | Radiation is the only mode that does NOT need a medium; it uses electromagnetic waves (that's how Sun's heat reaches Earth through vacuum). | | "Angle of incidence is measured from the mirror surface" | Always measure angles from the NORMAL (perpendicular line), not from the surface. | | "Light bends away from normal when entering glass from air" | Light bends TOWARD normal when entering a denser medium (glass from air). It bends away only when going from denser to rarer medium. | | "Sound travels faster in air than in water" | Sound travels FASTEST in solids, then liquids, then gases. Water carries sound about 4× faster than air. | | "Loudness and pitch are the same" | Loudness depends on amplitude (energy), pitch depends on frequency. A soft high note and a loud low note are different in both. | | "Ultrasound is dangerous sound" | Ultrasound simply means frequency above human hearing range; it is used safely in medical sonography and cleaning. |
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Quick Reference
- **Heat transfer modes**: Conduction (solids) → Convection (fluids) → Radiation (no medium)
- **Reflection law**: i = r (both from normal); image in plane mirror is virtual, erect, same size, laterally inverted
- **Refraction rule**: Denser medium → bends toward normal; Rarer medium → bends away
- **Sound speed order**: Solid > Liquid > Gas
- **Audible range**: 20 Hz – 20,000 Hz; below = infrasound, above = ultrasound
- **Echo minimum**: 17 m distance for distinct echo (persistence of sound = 0.1 s)