Heat, Light and Sound
Overview
Heat, Light and Sound form the core of classical physics tested in KTET Category II/III. The examiner tests your understanding of fundamental concepts, everyday applications, and the ability to connect scientific principles to classroom teaching situations.
For KTET success, you must grasp three distinct energy forms: thermal energy (heat), electromagnetic radiation (light), and mechanical waves (sound). Questions often involve temperature scales, modes of heat transfer, reflection and refraction of light, and characteristics of sound waves. Practical applications—thermos flasks, mirrors, lenses, musical instruments—are favourite areas for question setters.
Master the definitions precisely, understand the underlying physics of each phenomenon, and be ready to explain how you would demonstrate these concepts to upper primary or high school students.
Key Concepts
- Heat vs Temperature: Heat is the total kinetic energy of molecules (measured in Joules); temperature is the average kinetic energy (measured in °C, °F, or K). Heat flows from higher to lower temperature bodies.
- Three Modes of Heat Transfer: Conduction (through solids by molecular collision), convection (through fluids by bulk movement), and radiation (through electromagnetic waves, no medium needed).
- Laws of Reflection: Angle of incidence equals angle of reflection; incident ray, reflected ray, and normal lie in the same plane.
- Refraction and Snell's Law: Light bends when passing between media of different optical densities. The ratio of sines of angles equals the ratio of velocities (or refractive index).
- Sound as a Longitudinal Wave: Sound requires a medium; it travels as compressions and rarefactions. Speed is highest in solids, lowest in gases.
- Characteristics of Sound: Pitch depends on frequency, loudness on amplitude, and quality (timbre) on the waveform or harmonics present.
- Thermal Expansion: Solids, liquids, and gases expand on heating. Linear, areal, and volumetric expansion apply to solids; liquids show anomalous expansion (water is densest at 4°C).
- Total Internal Reflection: When light travels from denser to rarer medium at an angle greater than the critical angle, it reflects entirely back into the denser medium. Basis of optical fibres.
Formulas / Key Facts
| Concept | Formula / Fact |
|---|---|
| Temperature conversion | C/5 = (F − 32)/9 = (K − 273)/5 |
| Linear expansion | ΔL = L₀ × α × ΔT (α = coefficient of linear expansion) |
| Heat energy | Q = m × c × ΔT (c = specific heat capacity) |
| Latent heat | Q = m × L (L = latent heat of fusion/vaporisation) |
| Mirror formula | 1/f = 1/v + 1/u (f = focal length, v = image distance, u = object distance) |
| Lens formula | 1/f = 1/v − 1/u (sign convention: real is positive for convex lens) |
| Magnification | m = v/u = height of image / height of object |
| Snell's Law | n₁ sin i = n₂ sin r; n = c/v (refractive index) |
| Speed of sound | v = f × λ (f = frequency, λ = wavelength) |
| Speed of sound in air | Approximately 343 m/s at 20°C |
| Critical angle | sin C = 1/n (n = refractive index of denser medium w.r.t. rarer) |
| Doppler effect (sound) | Apparent frequency changes when source or observer moves |
Worked Examples
Example 1: Heat Calculation
A 500 g iron block is heated from 25°C to 75°C. Specific heat of iron = 0.45 J/g°C. Find heat absorbed.
Step 1: Identify values — m = 500 g, c = 0.45 J/g°C, ΔT = 75 − 25 = 50°C
Step 2: Apply Q = m × c × ΔT
Q = 500 × 0.45 × 50 = 11,250 J = 11.25 kJ
Example 2: Mirror Formula
An object is placed 30 cm from a concave mirror of focal length 15 cm. Find image position and nature.
Step 1: Use 1/f = 1/v + 1/u with sign convention (u = −30 cm, f = −15 cm for concave)
1/(−15) = 1/v + 1/(−30)
1/v = −1/15 + 1/30 = −2/30 + 1/30 = −1/30
v = −30 cm
Step 2: Image is at 30 cm on the same side as object → real, inverted, same size.
Example 3: Speed of Sound
A sound wave has frequency 256 Hz and wavelength 1.34 m. Find speed of sound.
v = f × λ = 256 × 1.34 = 343.04 m/s
This matches standard speed of sound in air at room temperature.
Common Mistakes
- Confusing heat and temperature → Heat is energy transferred; temperature is intensity measure. A large cold lake has more heat energy than a small cup of hot water.
- Applying mirror formula without sign convention → Always use consistent sign convention (New Cartesian). Object distance (u) is negative when object is in front of mirror.
- Thinking sound travels fastest in air → Sound travels fastest in solids (steel ~5000 m/s), then liquids, slowest in gases. Denser molecular packing = faster transmission.
- Mixing up concave and convex properties → Concave mirror converges light (used in torches, shaving mirrors); convex mirror diverges light (used in vehicle rear-view mirrors for wider field).
- Forgetting medium requirement for sound → Sound cannot travel through vacuum; light can. Classic question trap: "Why can't astronauts hear each other in space without radio?"
- Confusing real and virtual images → Real images can be projected on screen (formed by actual convergence); virtual images cannot be projected (formed by apparent divergence).
Quick Reference
- Heat flows from hot to cold; temperature measures hotness, heat measures energy.
- Conduction = solids, Convection = fluids, Radiation = no medium needed.
- Mirror: 1/f = 1/v + 1/u; Lens: 1/f = 1/v − 1/u.
- Refractive index n = sin i / sin r = speed in vacuum / speed in medium.
- Sound speed: solids > liquids > gases; approximately 343 m/s in air at 20°C.
- Pitch ↔ frequency; Loudness ↔ amplitude; Quality ↔ waveform.