Wave Behaviour
Section: Waves | Syllabus: Cambridge AS Level Physics 9702
Wave Behaviour When waves encounter obstacles or changes in medium, they exhibit fascinating behaviours. Understanding these is crucial for explaining everything from echoes to rainbows! Reflection Reflection When a wave bounces back after hitting a barrier or boundary.
Law of Reflection: The angle of incidence equals the angle of reflection Both angles are measured from the normal (an imaginary line perpendicular to the surface) The wavelength and frequency remain unchanged Law of Reflection: i = r Angle of incidence = Angle of reflection Real-world Examples: Light reflecting off mirrors → seeing your reflection Sound reflecting off walls → echoes in a canyon Radar waves bouncing off aircraft Ultrasound reflecting off organs in medical imaging Refraction Refraction The change in direction of a wave when it passes from one medium to another, caused by a change in wave speed.
What Happens During Refraction: Wave speed changes (increases or decreases) Wavelength changes (longer or shorter) Frequency stays the same Direction changes (unless entering perpendicular to the boundary) Key Rules: Wave entering a denser medium → slows down, bends toward the normal Wave entering a less dense medium → speeds up, bends away from the normal If wave enters perpendicular (90°), no change in direction (but speed and wavelength still change) Memory Trick "Fast Away, Slow Toward" - When wave speeds up → bends away from normal.
When wave slows down → bends toward normal. Diffraction Diffraction The spreading out of waves when they pass through a gap or around an obstacle. Diffraction is most noticeable when the gap width is similar to (or smaller than) the wavelength Smaller gap → more spreading Wavelength closer to gap size → greater diffraction Frequency and wavelength remain unchanged Important Why Can You Hear Around Corners But Not See?
Sound has much longer wavelengths (~cm to m) than light (~nanometers), so sound diffracts easily around everyday obstacles like doorways. Light wavelengths are too small compared to obstacles, so diffraction is minimal.
Interference (Superposition) Interference When two or more waves meet and overlap, their displacements combine according to the principle of superposition. Types of Interference: 1. Constructive Interference Waves meet in phase (crests align with crests) Displacements add together Resultant wave has larger amplitude Effect: Louder sound, brighter light, bigger water wave 2.
Destructive Interference Waves meet out of phase (crest aligns with trough) Displacements cancel out Resultant wave has smaller amplitude (can be zero) Effect: Quieter sound, dimmer light, flatter water Real-world Examples: Noise-cancelling headphones → use destructive interference to cancel unwanted sounds Dead spots in rooms → where sound waves destructively interfere Colorful patterns on soap bubbles and oil films → light interference Two speakers playing same tone → loud and quiet spots depending on interference Wave Behaviours Summary Behaviour What Happens Key Point Example Reflection Wave bounces back Angle in = Angle out Echoes, mirrors Refraction Wave bends at boundary Speed and wavelength change Lens, prism Diffraction Wave spreads through gap Greatest when gap ≈ wavelength Sound around corners Interference Waves combine Can add or cancel Noise-cancelling, soap bubbles Exam Success Strategy: For reflection questions: Always draw the normal and measure angles from it For refraction: Remember frequency never changes, only speed and wavelength For diffraction: Mention gap size relative to wavelength For interference: Clearly state whether waves are in phase or out of phase
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