Introduction
Light, Sound, and Heat are three forms of energy transfer tested heavily in Railway exams — this chapter
covers the core laws of reflection and refraction, sound wave characteristics, and the three modes of
heat transfer.
Reflection of Light
| Law |
|---|
| The angle of incidence equals the angle of reflection |
| The incident ray, reflected ray, and normal all lie in the same plane |
| Mirror Type | Image Formed | Use |
|---|
| Plane Mirror | Virtual, erect, same size | Everyday mirrors |
| Concave Mirror | Real or virtual, depending on object distance | Shaving mirrors, headlights, telescopes |
| Convex Mirror | Virtual, erect, diminished | Vehicle rear-view mirrors (wider field of view) |
Refraction of Light
📌 Key Concept: Refraction is the bending of light as it passes from one medium to another due to a change in speed — light bends towards the normal when entering a denser medium, and away from the normal when entering a rarer medium.
| Lens Type | Shape | Use |
|---|
| Convex Lens | Thicker at the centre (converging) | Magnifying glass, hypermetropia correction |
| Concave Lens | Thinner at the centre (diverging) | Myopia (short-sightedness) correction |
Dispersion and the Electromagnetic Spectrum
- Dispersion — splitting of white light into its 7 constituent colours (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red) when passed through a prism, due to different wavelengths bending at different angles.
- Rainbow formation — caused by dispersion, refraction, and total internal reflection of sunlight through water droplets in the atmosphere.
Sound — Properties of Waves
| Property | Description |
|---|
| Frequency | Number of vibrations per second; determines pitch. Unit: Hertz (Hz) |
| Amplitude | Maximum displacement from the rest position; determines loudness |
| Wavelength | Distance between two consecutive compressions (or rarefactions) |
| Speed of Sound | ~343 m/s in air (at room temperature); faster in liquids, fastest in solids |
⚠️ Common Trap: Sound needs a material medium to travel and CANNOT travel through a vacuum — unlike light, which can. This distinction is a frequent exam question.
Audible Range and Ultrasound
| Range | Frequency |
|---|
| Human audible range | 20 Hz to 20,000 Hz (20 kHz) |
| Infrasonic sound | Below 20 Hz |
| Ultrasonic sound | Above 20,000 Hz — used in SONAR, medical imaging (ultrasound) |
Heat — Modes of Transfer
| Mode | Description | Medium Needed |
|---|
| Conduction | Heat transfer through direct contact, molecule to molecule | Solids (best conductors: metals) |
| Convection | Heat transfer via movement of fluid (liquid/gas) particles | Liquids and gases |
| Radiation | Heat transfer via electromagnetic waves, no medium needed | Can travel through vacuum (e.g., Sun's heat reaching Earth) |
Q. Which mode of heat transfer allows the Sun's heat to reach the Earth through the vacuum of space?
Since no medium exists in space, heat cannot travel by conduction or convection
Answer: Radiation (the only mode that doesn't require a medium)
Temperature Scales
| Scale | Freezing Point of Water | Boiling Point of Water |
|---|
| Celsius | 0°C | 100°C |
| Fahrenheit | 32°F | 212°F |
| Kelvin | 273 K | 373 K |
✅ Exam Focus: Laws of reflection and mirror-type image characteristics · Refraction and convex/concave lens uses · VIBGYOR dispersion · Sound needs a medium (unlike light) · Three heat transfer modes and which one works in vacuum · Temperature scale conversions.