Unit 2: Waves
Waves transfer energy from one place to another without transferring matter. There are two primary types based on how particles move relative to the wave's direction of travel:
Transverse Waves: Particles move PERPENDICULAR (at right angles) to the direction the wave travels. Examples: light waves, ripples on water, waves on a guitar string.
Longitudinal Waves: Particles move PARALLEL (back and forth in the same direction) as the wave travels. Example: sound waves in air.
Transverse wave components:
• Crest: the highest point of the wave above equilibrium
• Trough: the lowest point of the wave below equilibrium
• Amplitude: the distance from the equilibrium (rest) position to a crest or trough — measures the wave's energy/intensity
• Wavelength (λ): the distance between two identical points on consecutive waves (e.g., crest to crest, or trough to trough)
• Equilibrium / rest position: the undisturbed baseline of the medium
Longitudinal wave components:
• Compression: a region where particles are pushed closely together (high pressure)
• Rarefaction: a region where particles are spread apart (low pressure)
Standing wave components:
• Nodes: points of zero displacement (no movement) — where the wave appears stationary
• Antinodes: points of maximum displacement — where the wave moves most
Wave measurements:
• Speed: how fast the wave travels through a medium
• Frequency: how many wave cycles pass a point per second (Hz)
• Period: the time for one complete wave cycle (seconds)
Waves interact with their environment in several ways:
Absorption: The wave's energy is taken in by the medium, reducing the wave's amplitude. Example: Sound is absorbed by foam.
Reflection: The wave bounces off a surface and travels back. Example: Echo of sound off a wall.
Refraction: The wave changes direction when it passes from one medium into another (where speed changes). Example: Light bending through water, creating a straw-in-water optical illusion.
Diffraction: The wave bends and spreads out around obstacles or through openings. Greatest when the opening/obstacle size ≈ wavelength. Example: Sound bending around corners.
Interference: Two or more waves overlap in the same space at the same time.
• Constructive interference: crests align with crests — amplitudes ADD, creating a larger wave
• Destructive interference: crests align with troughs — amplitudes SUBTRACT, reducing or canceling the wave
Doppler Effect: An apparent change in frequency due to relative motion between source and observer. Source moving TOWARD you → higher frequency (higher pitch). Source moving AWAY → lower frequency (lower pitch). Example: ambulance siren changing pitch.
A standing wave appears to be stationary — it doesn't seem to travel. It is created when a wave and its reflection interfere with each other consistently.
Key features:
• Nodes: fixed points of zero movement (destructive interference zones)
• Antinodes: points of maximum movement (constructive interference zones)
• Number of nodes: the 1st harmonic (fundamental) has 2 nodes (at the ends), 2nd harmonic has 3 nodes, etc.
Note: You will NOT be tested on calculating wavelength of standing waves — only on identifying and counting nodes.
The double-slit experiment demonstrates that light (and other waves) shows wave behavior — specifically, diffraction and interference working together.
How it works:
1. A wave (light) passes through two narrow parallel slits
2. Each slit acts as a new wave source — the waves DIFFRACT (spread out) as they pass through
3. The two diffracted waves then INTERFERE with each other
4. This creates an interference pattern: alternating bright bands (constructive interference, crests + crests) and dark bands (destructive interference, crests + troughs) on a screen
Significance: This result proves light behaves as a wave — a particle would not create this pattern. It was historically important evidence for the wave nature of light.