Heat, Light and Sound

Overview

Heat, Light and Sound form the core of classical physics at the upper-primary level and are consistently tested in MP TET Varg-2 (Mathematics and Science). These three forms of energy govern everyday phenomena—from cooking food to seeing objects to hearing music—making them highly relatable for classroom teaching and highly scorable in exams.

For MP TET, you must master three key areas: (1) modes of heat transfer and their practical applications, (2) behaviour of light during reflection and refraction including mirror and lens formulae, and (3) characteristics of sound waves and the human hearing mechanism. Questions often combine conceptual understanding with numerical problems involving mirror/lens equations or wave calculations. A strong grasp here also supports the pedagogy section, where you may be asked how to demonstrate these concepts through experiments.

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Key Concepts

• **Heat is energy in transit** due to temperature difference; it flows from higher to lower temperature until thermal equilibrium is reached.

• **Three modes of heat transfer**: Conduction (through solids via molecular vibration), Convection (through fluids via bulk movement), Radiation (through electromagnetic waves, no medium required).

• **Light travels in straight lines** (rectilinear propagation) at approximately 3 × 10⁸ m/s in vacuum; this explains shadows and eclipses.

• **Reflection obeys two laws**: angle of incidence equals angle of reflection, and incident ray, reflected ray and normal lie in the same plane.

• **Refraction occurs** when light changes speed as it passes from one medium to another, bending toward the normal when entering a denser medium.

• **Sound is a longitudinal mechanical wave**; it requires a material medium and cannot travel through vacuum.

• **Characteristics of sound**: Loudness (amplitude), Pitch (frequency), Quality/Timbre (waveform). Human audible range is 20 Hz to 20,000 Hz.

• **Echo and reverberation**: Echo requires minimum distance of 17.2 m (at 20°C) for distinct reflection; reverberation is multiple reflections causing sound persistence.

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Formulas / Key Facts

| Topic | Formula / Fact | |-------|----------------| | Heat absorbed/released | Q = m × c × ΔT (Q = heat, m = mass, c = specific heat, ΔT = temperature change) | | Mirror formula | 1/v + 1/u = 1/f (v = image distance, u = object distance, f = focal length) | | Magnification (mirror) | m = –v/u = height of image / height of object | | Lens formula | 1/v – 1/u = 1/f (sign convention: real is positive for convex lens) | | Power of lens | P = 1/f (f in metres); unit is Dioptre (D) | | Refractive index | n = speed in vacuum / speed in medium = sin i / sin r (Snell's Law) | | Speed of sound | v = f × λ (v = velocity, f = frequency, λ = wavelength) | | Speed of sound in air | Approximately 340 m/s at 20°C; increases with temperature | | Echo condition | Minimum distance = (speed × time) / 2; for t = 0.1 s, d = 17 m (approx 17.2 m) | | Audible frequency range | 20 Hz – 20,000 Hz; below 20 Hz = infrasonic, above 20,000 Hz = ultrasonic |

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Worked Examples

### Example 1: Heat Calculation **Problem**: How much heat is required 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
• Q = m × c × ΔT = 2 × 4200 × 50 = 4,20,000 J = 420 kJ

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### Example 2: Concave Mirror **Problem**: An object is placed 30 cm from a concave mirror of focal length 15 cm. Find the image position and magnification.

**Solution**:

• Sign convention: u = –30 cm, f = –15 cm (concave mirror, real focus)
• Mirror formula: 1/v + 1/u = 1/f
• 1/v + 1/(–30) = 1/(–15)
• 1/v = –1/15 + 1/30 = (–2 + 1)/30 = –1/30
• v = –30 cm (negative means real image, same side as object)
• Magnification m = –v/u = –(–30)/(–30) = –1
• Image is real, inverted and same size as object.

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### Example 3: Sound Wave **Problem**: A sound wave has frequency 512 Hz. If speed of sound in air is 340 m/s, find the wavelength.

**Solution**:

• v = f × λ → λ = v/f = 340/512 = 0.664 m ≈ 66.4 cm

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Common Mistakes

| Wrong Thinking | Correct Fix | |----------------|-------------| | Confusing heat and temperature—thinking they are the same. | Heat is energy transferred; temperature is the measure of average kinetic energy. A large lake at 25°C has more heat energy than a cup of water at 50°C. | | Forgetting sign conventions in mirror/lens problems, leading to wrong answers. | Always apply the New Cartesian Sign Convention: distances measured against incident light are negative; along incident light are positive. | | Assuming light bends away from normal when entering a denser medium. | Light bends toward the normal when entering a denser medium (higher refractive index) because it slows down. | | Believing sound travels fastest in air because we hear it easily. | Sound travels fastest in solids (e.g., ~5000 m/s in steel), slower in liquids (~1500 m/s in water), slowest in gases (~340 m/s in air). | | Using 1/v + 1/u = 1/f for lenses instead of 1/v – 1/u = 1/f. | Mirror formula uses addition; lens formula uses subtraction (1/v – 1/u = 1/f). Remember: "Lens has a minus." | | Thinking echo and reverberation are identical phenomena. | Echo is a distinct repeated sound (needs ~17 m distance); reverberation is prolonged sound due to multiple quick reflections in enclosed spaces. |

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Quick Reference

• **Heat transfer modes**: Conduction (solids) → Convection (fluids) → Radiation (no medium).
• **Mirror formula**: 1/v + 1/u = 1/f; Lens formula: 1/v – 1/u = 1/f.
• **Snell's Law**: n = sin i / sin r; light bends toward normal in denser medium.
• **Sound speed ranking**: Solid > Liquid > Gas; ~340 m/s in air at 20°C.
• **Audible range**: 20 Hz – 20,000 Hz; below = infrasonic, above = ultrasonic.
• **Echo minimum distance**: ~17 m (sound needs 0.1 s to return for distinct perception).