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IB DP Physics — Unit C: Wave Behaviour | SL & HL | C.3 Wave Phenomena
📋 What You Will Learn
- Observe and describe reflection, refraction, and diffraction using real-world examples and wave diagrams
- Define the law of reflection, Snell's law, and the conditions for total internal reflection
- Derive Snell's law from the geometry of wavefronts and the definition of refractive index
- Apply Snell's law and the critical angle equation to solve figure-based and calculation problems
- Analyse how slit width and wavelength determine the extent of diffraction
- Evaluate common misconceptions and connect wave phenomena to optics, fibre optics, and the wider IB Physics course
🌍 Real-World Context Look at a swimming pool from the side: the bottom appears shallower than it really is, and a straight stick inserted into the water appears to bend at the surface. At the same time, light bouncing off the water's surface creates shimmering reflections on nearby walls. Meanwhile, sound from a distant conversation seems to bend around a corner, reaching your ears even though you cannot see the speaker. These three everyday observations — the bending of light at a boundary, the bouncing of waves off a surface, and the spreading of waves around obstacles — are all manifestations of the same underlying wave physics. Understanding them unlocks the design of optical fibres, camera lenses, radio antennas, and medical imaging technologies.
🔍 Guiding Question How do waves behave when they encounter boundaries, changes in medium, or obstacles — and what determines the direction, speed, and spread of the wave after each interaction?
Part 1 — Reflection
Physical Setup: Waves Hitting a Boundary
Imagine ripples on a water surface travelling toward a straight wall. When the wavefronts reach the wall, they do not simply stop — they bounce back, continuing to travel but now in a new direction. This bouncing of waves off a surface is called reflection . The same phenomenon occurs with light hitting a mirror, sound echoing off a cliff, and radar pulses returning from an aircraft.
To describe reflection precisely, we need a consistent way to measure the direction of the incoming and outgoing waves. Physicists use the concept of a ray — a line drawn perpendicular to the wavefronts, showing the direction of wave travel — and a normal — a line drawn perpendicular to the reflecting surface at the point of incidence. All angles are measured from this normal, not from the surface itself.
Notice in the diagram that the incident ray, the normal, and the reflected ray all lie in the same plane. The angle of incidence \( \theta_i \) and the angle of reflection \( \theta_r \) are both measured from the normal. This geometric relationship is the same for all types of waves — light, sound, water waves, and microwaves all obey the same law of reflection.
Core Concepts: The Law of Reflection
When we measure \( \theta_i \) and \( \theta_r \) for many different angles of incidence, we find a remarkably simple and universal result: the two angles are always equal. This is not a coincidence — it follows directly from the wave nature of light and the requirement that wavefronts remain coherent after bouncing off a surface.
📖 Key Definition — Law of Reflection The angle of incidence equals the angle of reflection: \( \theta_i = \theta_r \). Both angles are measured from the normal to the reflecting surface at the point of incidence. The incident ray, the normal, and the reflected ray are coplanar (they all lie in the same plane).
Notation Summary — Reflection
\( \theta_i \)