Waves & Oscillations

  • Every oscillatory motion (small frequency) is vibratory motion (large frequency) but not vice a versa
  • SHM Force is directly proportional to displacement from mean / equilibrium position

F = -Kx   (Springs – Hooks law), Displacement is sinusoidal function of time

Frequency = 1 / time,     w (Angular frequency) = 2 (pi) / t,       T = 2 (pi) (L/g)1/2

Waves & Oscillations Hookes law spring

Light & sound wave form Transfer of energy without net transfer of matter

Mechanical waves

  • Waves which require a medium to propagate.
  • Cannot travel through vacuum
  • For ex. sound waves (longitudinal waves), Seismic waves, Water waves, Waves on a string etc.


Non Mechanical waves

  • Waves which do not require a medium to propagate (Electromagnetic waves).
  • Can even travel through vacuum
  • For ex. Radio waves, Light waves, X rays, UV waves, IR rays

Frequency = No. of waves passing through a point per unit second

Sound Waves

  • Longitudinal waves
  • Cannot travel through vacuum
  • Measured in decibels
  • Speed of sound = 330 m/s
  • No effect of pitch & loudness)
  • Denser material have high speed of sound waves in most cases
  • loudness Depends on amplitude & hence can be increased by setting a greater mass in vibration
  • Humans hearing frequency =  (16 – 20,000) Hz
  • f < 16 Hz (infrasonic)
  • f > 20000 Hz (Ultrasonic)
Speed of sound increases with increase in temp of air (due to fast movement of molecules) Vwarm air > Vcold air Speed of sound decreases with increase in altitude (due to lower no. of molecules available to displace)

Speed of sound is greater in water than in air hence Vhumid air > Vdry air

Echo (Reflection of sound)

  • To measure depth of sea beds using SONAR
  • To detect oil & gas underwater
  • Used for Ultrasonography



Natural frequencies of an object, when set in oscillation continuously leads to greater amplitudes even with smaller forces or oscillations, for ex.

  • Driver jumping on swimming pool jump boards continuously to gain more amplitudes
  • Soldiers walking slow on suspension bridges


Doppler Effect

  • Change in frequency of a wave (sound or light) due to motion of source or observer
  • Higher is the frequency when source is approaching the observer & vice a versa

Doppler effect

Examples are 

  • Whistle of a train
  • Speed guns used by traffic police
  • Star receding (Red shift) or approaching (Blue shift)
  • Rotation of star



  • Arise from the interference of waves
  • When 2 harmonic waves of close frequencies are heard at a same time, we hear audibly distinct waxing & waning intensity of sound
  • Artists tune their devices not to detect any beats


Electromagnetic Spectrum

Electromagnetic Spectrum

Radio waves  Produced by accelerated motion of charge in wiresUsed in Radio & TV communication systems, Cellular phones, MRI
 Microwaves  Used in Radar system;  Aircraft navigationSpeed guns in cricket, Tennis, AutomobilesOvens & Vacuumed tubes
 IR waves  Produced by hot bodies, also known as heat waves (Maintain earths warmth)Used as IR detectors in satellite for military & observing growth of cropsUsed in TV remotes & widely used in Photography.
Visible Rays  Visible range — Wavelength (700 – 400 nm)
 UV rays  Produced by very hot bodies ex. sunAbsorbed by ordinary glasses (Welders wear glass goggles to protect their eyes from harmful UV rays, produced during welding)UV lamps are used to kill germs in water purifiers
India Yearbook English India Yearbook Hindi Economic Survey 2017

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