Motion, Light and Sound
Overview
Motion, Light and Sound are fundamental physics topics frequently tested in SSC CGL and CHSL exams. Motion covers speed, velocity, acceleration and Newton's Laws. Light deals with reflection, refraction, lenses and the visible spectrum. Sound covers wave properties, speed in different media, and human hearing range. Mastering key formulas, definitions and numerical values from these three areas ensures high scoring in the General Science section.
Typed Notes
Motion: Types, Equations and Newton's Laws
Motion is the change in position of an object with time. Uniform motion means equal distances in equal time intervals; non-uniform motion means unequal distances. Three equations of motion: (1) v = u + at, (2) s = ut + ½at², (3) v² = u² + 2as. Newton's First Law states a body remains at rest or in uniform motion unless acted upon by an external force (Inertia). Second Law: Force = mass × acceleration (F = ma). Third Law: Action and reaction are equal and opposite. Momentum = mass × velocity (kg·m/s). Impulse = Force × Time.
Light: Reflection, Refraction and Lenses
Light travels in straight lines at 3 × 10⁸ m/s in vacuum. Laws of Reflection: angle of incidence = angle of reflection; incident ray, normal and reflected ray are coplanar. Concave mirrors converge light (used in torches, headlights, solar cookers). Convex mirrors diverge light and give a wider field of view (used in rear-view mirrors). Refraction is bending of light when it passes from one medium to another. Convex (converging) lens corrects hypermetropia (far-sightedness); Concave (diverging) lens corrects myopia (near-sightedness). Total Internal Reflection causes mirages and optical fibre working. VIBGYOR is the order of visible spectrum (Violet to Red).
Sound: Properties, Speed and Applications
Sound is a longitudinal mechanical wave requiring a medium to travel. Speed of sound: fastest in solids > liquids > gases. In air at 0°C it is 332 m/s; at 25°C approximately 346 m/s. Frequency determines pitch; amplitude determines loudness. Unit of loudness is Decibel (dB). Echo is reflected sound heard after 0.1 second; minimum distance for echo = 17 m. SONAR (Sound Navigation And Ranging) uses ultrasound to detect underwater objects. Ultrasound is used in medical ultrasonography. Infrasound (below 20 Hz) is produced by earthquakes and detected by elephants and whales. Resonance occurs when a body vibrates at its natural frequency.
Key Formulas and Exam-Ready Numbers
Memorise these values for SSC: Speed of light = 3 × 10⁸ m/s; Speed of sound in air = 332 m/s (0°C); Audible range = 20 Hz–20,000 Hz; Refractive index of glass ≈ 1.5; Refractive index of water ≈ 1.33; Power of lens = 1/focal length in metres (unit: Dioptre). Doppler Effect: frequency appears higher when source approaches, lower when it recedes (used in RADAR speed guns). Persistence of vision = 1/16th of a second (basis of cinema). Focal length of concave mirror is negative; convex mirror focal length is positive in sign convention.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Speed = Distance/Time; Velocity = Displacement/Time; Acceleration = Change in Velocity/Time — all SI units must be memorised (m/s, m/s²).
- 02Newton's First Law = Law of Inertia; Second Law: F = ma; Third Law: Every action has an equal and opposite reaction.
- 03Speed of light in vacuum = 3 × 10⁸ m/s; Speed of sound in air at 0°C = 332 m/s, increases with temperature and is fastest in solids.
- 04Refractive index (n) = Speed of light in vacuum / Speed of light in medium; higher n means denser medium and more bending of light.
- 05Human audible range = 20 Hz to 20,000 Hz; below 20 Hz = Infrasound; above 20,000 Hz = Ultrasound (used in SONAR and medical imaging).
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
A car starts from rest and reaches a velocity of 72 km/h in 10 seconds. What is its acceleration?
Why is a convex mirror preferred as a rear-view mirror in vehicles?
What is the minimum distance required between a person and a wall to hear a distinct echo? Why?
Which type of lens is used to correct myopia and which is used for hypermetropia? Name the defect and its cause.
State Newton's Second Law of Motion and explain how it differs from the First Law.
Motion, Light and Sound cover Newton's three laws, equations of motion, reflection and refraction of light, lens types and their uses, and sound wave properties. Key numbers: light speed = 3 × 10⁸ m/s, sound speed in air = 332 m/s, audible range = 20 Hz to 20,000 Hz. Concave lens corrects myopia; convex lens corrects hypermetropia. Echo needs minimum 17 m distance. These facts recur consistently across SSC CGL and CHSL papers.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.The speed of sound is maximum in which medium?
Q2.Which of the following correctly represents VIBGYOR in the visible spectrum from longest to shortest wavelength?
Q3.A body is said to be in uniform motion when it covers:
Q4.Ultrasound waves have a frequency:
Mark karo — dashboard par progress dikhega.