Waves
Class 11 · Physics
4 ideas in Class 11 Physics, explained in plain words, with games and quizzes to practise.
Transverse, Longitudinal and Travelling Waves
Part of: Kinds and Speeds of Waves
Sideways shakes make transverse waves; push-pull makes longitudinal. Only shear-proof solids carry both; air carries only sound. A travelling wave is a sine of position and time whose shape glides at frequency times wavelength.
Why it matters. Springs, air and water each carry their own kind. Reading the wave equation reveals speed, size and pace at once.
Formulas
- v = frequency x wavelength
- Wave form: y = A sin(kx - wt)
- k = 2 pi / wavelength; w = 2 pi x frequency
Key words
- Transverse wave
- : Sideways shake at right angles to travel.
- Longitudinal wave
- : Push-pull motion along travel, like sound in air.
- Amplitude
- : The tallest reach of the wave.
- Wave number
- : Waves per metre times two pi.
- Wavelength
- : Length of one full wave repeat.
Try it in two minutes
- Flick a rope sideways and along, comparing.
- Clap and feel sound arrive as pushes.
- Trace a sine and mark crest, trough, length.
Try at home. Make rope waves fast and slow, long and short.
Speed of Waves in Media
Part of: Kinds and Speeds of Waves
Strings race at root tension over mass per length. Sound cruises at root stiffness over density. Stiffer, lighter media run faster; the source sets frequency and the medium then fixes wavelength.
Why it matters. Tuning, sonar and quake timing all read these roots. Medium decides speed; source decides pitch.
Formulas
- String speed = root(T / mu)
- Sound speed = root(B / rho)
- v = frequency x wavelength links source to medium.
Key words
- Linear mass density
- : Mass per unit length of a string.
- Bulk modulus
- : Stiffness setting sound speed in fluids.
- Stiffness-to-density rule
- : Stiffer lighter media run waves faster.
- Medium-fixed speed
- : Speed set by the medium, not the source frequency.
Try it in two minutes
- Tighten a string and hear pitch climb.
- Compare sound in air versus water delays.
- Double frequency and watch wavelength halve.
Try at home. Pluck bands of different tightness and length.
Standing Waves and Harmonics
Part of: Standing Waves, Beats and Doppler
Meeting waves add peak to peak; reflections pin nodes at fixed ends. Strings sing whole-number modes with both ends nodes. Open pipes match strings; closed pipes keep only odd modes with a node at the shut end.
Why it matters. Music, halls and instruments tune these modes. Ends decide which notes survive.
Formulas
- String and open pipe: fn = n v / (2 L)
- Closed pipe: only odd modes, fn = n v / (4 L)
- Nodes sit half a wavelength apart.
Key words
- Standing wave
- : A still pattern from meeting waves.
- Node
- : A point that never moves.
- Antinode
- : A point swinging widest.
- Fundamental
- : The lowest allowed mode.
- Harmonic
- : A whole-number mode of the fundamental.
Try it in two minutes
- Shake a rope tied at one end and find still points.
- Blow across bottles of two depths.
- Hum with a tube open, then capped.
Try at home. Pluck rubber bands of two lengths and compare pitch.
Beats and the Doppler Effect
Part of: Standing Waves, Beats and Doppler
Near-equal tones wax and wane at their difference rate: beats. Motion squeezes waves ahead and stretches them behind, lifting pitch for approach and dropping it for retreat. The shift sizes with speed over sound speed.
Why it matters. Tuners count beats; horns, radars and galaxies report motion by pitch. One difference rule hears both.
Formulas
- Beats per second = f1 - f2 (difference)
- Approach: f = f0 x V / (V - vs)
- Retreat: f = f0 x V / (V + vs)
Key words
- Beats
- : Waxing at the difference rate of near-equal tones.
- Beat frequency
- : The difference of the two frequencies.
- Doppler effect
- : Pitch lift on approach, drop on retreat.
- Red shift
- : Waves stretched by a retreating source.
Try it in two minutes
- Strike two near forks and count the wobble.
- Listen to a passing horn's falling note.
- Walk toward sound and note the lift.
Try at home. Tune two strings to a slow beat, then to zero.
What your child will practise
- Beats and the Doppler Effect: Jungle Quick Fire (Quick Fire)
- Speed of Waves in Media: Candy Puzzle (Clue Puzzle)
- Speed of Waves in Media: Candy Quick Fire (Quick Fire)
- Standing Waves and Harmonics: Desert Quick Fire (Quick Fire)
- Transverse, Longitudinal and Travelling Waves: Space Puzzle (Clue Puzzle)
- Quiz: Beats and the Doppler Effect (8 questions)
- Quiz: Speed of Waves in Media (8 questions)
- Quiz: Standing Waves and Harmonics (8 questions)
- Quiz: Transverse, Longitudinal and Travelling Waves (8 questions)