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) |