Waves

Overview

This lecture covers the properties and types of waves, their characteristics, how they interact with matter, the electromagnetic spectrum, wave reflection and refraction, and image formation with lenses.

Types of Waves

• Waves transfer energy without transferring matter; oscillations pass along instead of particles.
• Longitudinal waves have oscillations parallel to energy transfer (e.g., sound waves, seismic P waves).
• Compressions are regions where particles bunch up; rarefactions are where they're spread out.
• Transverse waves have oscillations perpendicular to energy transfer (e.g., water waves, seismic S waves, light).

Wave Properties & Measurement

• Displacement (y-axis) is how far particles move from their rest position.
• Amplitude is the maximum displacement from equilibrium.
• Wavelength (λ) is the distance of one complete wave, measured in meters.
• Time period (T) is the time for one wave to pass, measured in seconds.
• Frequency (f) is the number of waves passing a point per second, measured in Hertz (Hz).
• Frequency and time period are reciprocals: f = 1/T.
• Wave speed equation: v = f × λ.

Measuring Waves

• Use a ripple tank to find frequency and wavelength; then calculate speed.
• Measure speed by timing wave travel over distance: speed = distance/time.
• Measure sound speed using a microphone and oscilloscope, timing echoes.

Sound, Hearing, and Ultrasound

• Sound makes the eardrum vibrate, converting to signals sent to the brain.
• Human hearing range: 20 Hz to 20 kHz; above this is called ultrasound.
• Ultrasound is partially transmitted and reflected at material boundaries; echoes are used for medical imaging and sonar.

Seismic Waves and Earth's Structure

• P waves (longitudinal) travel through liquids; S waves (transverse) do not.
• Lack of S wave aftershocks on opposite sides of Earth suggests a liquid core.

Reflection and Refraction

• Specular reflection: smooth surfaces reflect waves at equal incidence and reflection angles (measured from the normal).
• Diffuse reflection: rough surfaces scatter waves.
• Refraction is the change in wave direction when entering a new medium; light slows and bends closer to the normal when entering denser materials.

Electromagnetic (EM) Waves

• EM waves don't need a medium and can travel through a vacuum.
• EM spectrum: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
• Higher frequency EM waves carry more energy; gamma rays are emitted by nuclei.
• Ionizing EM waves (UV, X-rays, gamma) can damage cells and DNA.
• EM waves are used in communication, heating, imaging, and medical treatments.

Lenses and Image Formation

• Convex lenses converge light rays; principal focus is where parallel rays meet.
• Real images can be projected; they're inverted and can be diminished.
• Closer objects form virtual, magnified, upright images (e.g., magnifying glass).
• Concave lenses diverge rays, always forming diminished, upright, virtual images.
• Magnification = image height/object height.

Color and Black Bodies

• Color is perceived from wavelengths reflected/emitted and absorbed by the retina.
• Objects absorb some wavelengths, reflecting others (leaves appear green, blue balls reflect blue).
• A perfect black body absorbs and emits all wavelengths; real objects approximate this.

Key Terms & Definitions

• Longitudinal Wave — a wave with oscillations parallel to energy transfer.
• Transverse Wave — a wave with oscillations perpendicular to energy transfer.
• Wavelength (λ) — distance between successive wave crests, in meters.
• Frequency (f) — waves per second at a point, measured in Hertz (Hz).
• Amplitude — maximum displacement from equilibrium.
• Reflection — bouncing of waves off a surface.
• Refraction — bending of waves when entering a new medium.
• Specular Reflection — reflection from a smooth surface.
• Diffuse Reflection — scattering from a rough surface.
• Principal Focus — point where rays converge after passing through a lens.
• Black Body — idealized object absorbing/emitting all radiation wavelengths.

Action Items / Next Steps

• Practice identifying wave types and properties.
• Use the wave equation to solve speed, frequency, and wavelength problems.
• Review the EM spectrum and associated uses/dangers.
• Prepare for practicals on measuring wave properties and effects of lenses.