Overview
This lecture covers the properties, types, and behaviors of waves, with a focus on electromagnetic and sound waves, including key phenomena such as reflection, refraction, total internal reflection, and the Doppler effect.
Wave Basics
- Waves transfer energy without transferring matter.
- Two types of waves: transverse (vibration perpendicular to propagation) and longitudinal (vibration parallel to propagation).
Transverse and Longitudinal Waves
- Transverse wave examples: water waves, seismic S-waves, electromagnetic waves.
- In transverse waves, crests are peaks and troughs are valleys.
- Longitudinal wave examples: sound waves, seismic P-waves.
- Compressions (high pressure) and rarefactions (low pressure) occur in longitudinal waves.
Wave Properties
- Amplitude: maximum displacement from equilibrium, relates to energy carried.
- Wavelength (λ): distance between two consecutive identical points (e.g. crests), measured in meters.
- Frequency (f): number of waves passing a point per second, measured in hertz (Hz).
- Period (T): time for one vibration or wave, measured in seconds (s), with f = 1/T.
- Speed (v): wavefront distance per time, v = f × λ.
Wave Phenomena
- Wavefronts: lines joining points vibrating in phase (crests or compressions).
- Reflection: angle of incidence equals angle of reflection, wavelength and speed unchanged.
- Refraction: wave speed and wavelength change across mediums, frequency remains constant, bending occurs toward or away from normal depending on speed change.
- Doppler Effect: apparent change in frequency/wavelength due to moving source (higher pitch in front, lower behind).
Electromagnetic Waves
- All are transverse, travel at 3 × 10⁸ m/s in vacuum, and can travel through a vacuum.
- Spectrum (longest to shortest wavelength): radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
- Higher frequency means higher energy and more ionization risk (dangerous: UV, X-rays, gamma rays; safer: radio, microwave, IR, visible).
Uses and Risks of Electromagnetic Waves
- Applications: communication (radio, microwave, fiber optics), cooking (microwave), medical imaging (X-rays, gamma), security (X-rays), sterilization (UV, gamma).
- Hazards: high-energy waves can cause cell mutation and cancer; lower energy can cause tissue heating or eye damage.
Light Wave Behaviour
- Light is a transverse electromagnetic wave travelling at 3 × 10⁸ m/s.
- Reflection on mirrors: angle of incidence = angle of reflection.
- Refraction: light slows and bends toward normal in denser media; refractive index n = sin i / sin r.
- Total internal reflection: occurs when traveling from denser to less dense medium and the angle exceeds the critical angle (c = 1/n).
Uses of Reflection and Refraction
- Periscopes, binoculars, rear reflectors, and optical fibers use total internal reflection for efficient light transmission and viewing.
Sound Waves
- Sound waves are longitudinal and need a medium to travel.
- Speed of sound: ~330 m/s in air, faster in liquids (~1,500 m/s) and solids (~3,000–5,000 m/s).
- Loudness depends on amplitude, pitch on frequency.
- Humans hear 20 Hz – 20,000 Hz; above is ultrasound, below is infrasound.
- Reflected sound produces echoes, used in sonar and echolocation.
Measuring Sound
- Speed of sound: distance divided by time; can use echoes for calculation.
- Oscilloscope connected to a microphone visualizes sound waves as transverse, making amplitude and frequency measurable.
Key Terms & Definitions
- Transverse Wave — vibration perpendicular to wave travel direction.
- Longitudinal Wave — vibration parallel to wave travel direction.
- Amplitude — maximum displacement from equilibrium.
- Wavelength (λ) — distance between identical points on a wave.
- Frequency (f) — number of waves per second.
- Period (T) — time for one complete cycle of vibration.
- Wavefront — line connecting points in phase on a wave.
- Reflection — wave bouncing off a surface.
- Refraction — bending of waves at a boundary due to speed change.
- Doppler Effect — frequency/wavelength change due to moving source.
- Refractive Index (n) — ratio describing how much a material bends light.
- Critical Angle (c) — minimum incidence angle for total internal reflection.
- Total Internal Reflection — reflection of all light at boundary in denser medium.
- Ultrasound — sound waves above 20,000 Hz.
Action Items / Next Steps
- Review and practice drawing diagrams of wave reflection and refraction.
- Memorize wave equations and electromagnetic spectrum order.
- Complete assigned textbook reading on wave phenomena and applications.