Wave: a mechanism that transfers energy without transferring the medium.
Mechanical waves need a medium (sound, water, string). Electromagnetic waves need no medium (light, radio).
Transverse wave: particles vibrate perpendicular to the direction of the wave (light, water, string). Longitudinal wave: particles vibrate parallel to the direction (sound, spring).
Amplitude: maximum displacement from the mean position. Wavelength (λ): distance between two successive crests (or troughs). Frequency (f): vibrations per second, Hz. Time period (T): time for one vibration, seconds.
Reflection: wave bounces back from a surface. Refraction: change of direction when speed changes on entering another medium. Diffraction: bending of waves around the edges of an obstacle or through a gap.
Simple harmonic motion: vibratory motion in which acceleration is proportional to displacement and directed to the mean position. Damped oscillation: amplitude decreases with time because energy is lost to friction.
Formulas
v = f λ, T = 1/f, f = number of waves / time. For EM waves in vacuum c = 3 × 10⁸ m/s. Wave energy is proportional to (amplitude)².
Must-know points
Ripple tank: electrical vibrator makes waves; frequency = 1/T of vibrator. Decreasing T raises frequency; speed in the same depth stays constant, so wavelength decreases.
Deep to shallow water: speed decreases, wavelength decreases, frequency and time period do not change; waves bend towards the normal (refraction, like light entering glass).
Reflection: angle of incidence = angle of reflection; speed, frequency and wavelength unchanged.
Diffraction is maximum when the gap width is about equal to the wavelength. Smaller gap or longer wavelength gives more diffraction. Example: radio waves reach valleys; we hear sound around corners.
Sun fusion sound: we feel heat (EM radiation) but hear nothing because sound is mechanical and cannot cross vacuum.
Seismic waves: P waves are longitudinal and faster, travel through solids and liquids; S waves are transverse, slower, only through solids. Tsunamis: huge ocean waves from underwater earthquakes, landslides or eruptions; they slow down and grow taller in shallow water.
Energy transfer in a transverse wave: each particle passes energy to the next while it only moves up and down, perpendicular to the direction of energy flow. Draw a wave form with crest, trough, amplitude and wavelength labelled.
Past-paper questions with model answers
What is time period? Write its SI unit: time for one complete vibration, second (s).
Damped oscillation with sketch: oscillation whose amplitude gradually decreases; sketch a sine wave with shrinking amplitude.
How is energy transferred in a transverse wave? Make the wave form (3 marks).
Draw wave forms to compare vibration directions of transverse and longitudinal waves (1.5 + 1.5).
Diffraction of water waves with the ripple tank (Section C): plane waves pass through a gap; wide gap gives little bending; gap near wavelength gives circular waves; draw both diagrams.
How does wavelength or gap size affect diffraction? Longer wavelength or narrower gap gives more bending.
200 waves in 40 s, wavelength 4 cm: f = 200/40 = 5 Hz, T = 0.2 s, v = fλ = 5 × 0.04 = 0.2 m/s. 50 Hz and 2 mm: v = 50 × 0.002 = 0.1 m/s.
Radio wave of 90 MHz: λ = 3 × 10⁸ / 90 × 10⁶ = 3.33 m.