Light, Sound and Heat
Overview
Light, Sound and Heat are fundamental topics in General Science for Railway NTPC and Group D exams. Light travels as electromagnetic waves, sound as mechanical waves, and heat transfers through conduction, convection and radiation. Questions focus on speed values, reflection/refraction laws, properties of sound, types of mirrors and lenses, and heat transfer modes. Mastering units, formulas and real-life applications gives a strong scoring edge in these exams.
Typed Notes
Light — Nature, Reflection and Refraction
Light is an electromagnetic wave that travels at 3 × 10⁸ m/s in vacuum. It does NOT need a medium. Laws of reflection: angle of incidence = angle of reflection; incident ray, reflected ray and normal are coplanar. Refraction is bending of light when it passes from one medium to another. Refractive index (n) = speed of light in vacuum ÷ speed in medium. Total Internal Reflection causes mirages and optical fibre working. A prism splits white light into VIBGYOR (seven colours). Red light deviates least; Violet deviates most.
Mirrors and Lenses — Types and Uses
Concave (converging) mirror: reflecting surface curves inward; used in torches, solar cookers, ENT doctors' head mirrors, shaving mirrors. Convex (diverging) mirror: curves outward; used as rear-view/side-view mirror in vehicles because it gives wider field of view. Convex lens (converging): used to correct Hypermetropia (far-sightedness). Concave lens (diverging): used to correct Myopia (near-sightedness). Bifocal lens corrects both. Power of lens = 1/focal length (in metres); unit is Dioptre (D). Exam tip: Concave = Cave = curves IN.
Sound — Properties, Echo and Ultrasound
Sound is a mechanical, longitudinal wave needing a material medium. Speed: fastest in solids > liquids > gases. Speed in steel ≈ 5100 m/s; in water ≈ 1500 m/s; in air ≈ 346 m/s. Echo requires minimum distance of 17 m from reflecting surface (at 25°C). Pitch depends on frequency; loudness depends on amplitude. Ultrasound (>20,000 Hz) is used in SONAR, medical imaging (USG), cleaning jewellery, detecting cracks in metals. Infrasound (<20 Hz) is produced by earthquakes, whales. Unit of sound intensity = Decibel (dB).
Heat — Transfer, Specific Heat and Effects
Heat is a form of energy measured in Joules; temperature is measured in Kelvin (SI), Celsius or Fahrenheit. Conversion: °C = (°F − 32) × 5/9; K = °C + 273. Conduction: heat transfer through direct contact in solids; metals are good conductors. Convection: transfer through fluid movement; responsible for sea breeze, land breeze, trade winds. Radiation: transfer without medium; dark/rough surfaces absorb and emit more radiation (good absorbers = good emitters — Kirchhoff's Law). Specific heat of water = 4200 J/kg·K — highest among common substances, making water ideal coolant.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Speed of light in vacuum = 3 × 10⁸ m/s; Speed of sound in air at 0°C = 332 m/s, at 25°C ≈ 346 m/s
- 02Sound requires a material medium to travel; it CANNOT travel through vacuum — light can
- 03Concave mirror is used in torches, headlights, shaving mirrors; Convex mirror is used as rear-view mirror in vehicles
- 04Frequency of sound below 20 Hz = Infrasound; above 20,000 Hz = Ultrasound; human hearing range = 20 Hz to 20,000 Hz
- 05Heat transfers by Conduction (solids), Convection (liquids/gases), and Radiation (no medium needed — e.g., Sun's heat reaching Earth)
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
Why is a convex mirror preferred as a rear-view mirror in vehicles?
What is the minimum distance required from a wall to hear an echo, and why?
Differentiate between Myopia and Hypermetropia and state the corrective lens used for each.
State three modes of heat transfer with one real-life example each.
What is SONAR and on which property of sound does it work?
Light (electromagnetic, 3×10⁸ m/s), Sound (mechanical, needs medium, 346 m/s in air) and Heat (transfers via conduction, convection, radiation) are core Railway exam topics. Key facts: concave mirror for torches, convex mirror for vehicles, ultrasound in SONAR/USG, echo needs 17 m distance, water has highest specific heat, and speed of sound is fastest in solids. Memorise units — Decibel, Dioptre, Kelvin — for direct MCQ gains.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which of the following cannot travel through vacuum?
Q2.The speed of light in vacuum is approximately:
Q3.Which mirror is used by an ENT (ear, nose, throat) doctor to examine patients?
Q4.The SI unit of temperature is:
Mark karo — dashboard par progress dikhega.