C.3
Wave Phenomena
Reflection, refraction, diffraction and superposition for SL. HL adds single-slit diffraction patterns, double-slit modulation and diffraction gratings.
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C.3 Wave Phenomena
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C.3 Wave Phenomena — SL
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Key Concepts, Wave Phenomena
Wavefronts, Rays and Wave Behaviour at Boundaries
A wavefront is a line or surface connecting all points that are in phase (e.g. all the crests of a wave). Rays are lines drawn perpendicular to wavefronts, showing the direction of wave travel. At a boundary between two media, three things happen simultaneously: reflection (wave bounces back into the original medium), refraction (wave passes into the new medium, changing speed and direction), and transmission (energy continues into the new medium). The angle of incidence equals the angle of reflection. When a wave slows down entering a new medium, it bends towards the normal; when it speeds up, it bends away from the normal.
Snell's Law and Total Internal Reflection
Snell's law relates the angles of incidence and refraction to the refractive indices of the two media: n₁ sinθ₁ = n₂ sinθ₂, where n = c/v is the refractive index (ratio of speed of light in vacuum to speed in the medium). When light passes from a denser to a less dense medium (e.g. glass to air), it bends away from the normal. If the angle of incidence exceeds the critical angle θ_c = arcsin(n₂/n₁), the refracted ray would have to travel along the boundary or beyond, so all the light is reflected back into the denser medium. This is total internal reflection (TIR), used in optical fibres, periscopes and diamond cutting.
Diffraction
Diffraction is the spreading of a wave as it passes through an aperture or around an obstacle. The effect is most pronounced when the wavelength is similar in size to the gap or obstacle. A narrow gap produces significant spreading; a wide gap produces little spreading (the wave passes almost straight through). This is why sound (long wavelengths) diffracts easily around everyday objects, but visible light (short wavelengths) requires very narrow slits to show noticeable diffraction.
Superposition and Double Source Interference
The principle of superposition states that when two waves meet, the resultant displacement is the sum of the individual displacements. Constructive interference occurs when two waves arrive in phase (path difference = nλ, where n is an integer): the amplitudes add to give a maximum. Destructive interference occurs when two waves arrive in antiphase (path difference = (n + ½)λ): the amplitudes cancel to give a minimum. For two coherent sources (same frequency, constant phase difference) separated by distance d, bright fringes are spaced s = λD/d apart on a screen at distance D. Young's double slit experiment first proved that light behaves as a wave.
Single Slit Diffraction and Diffraction Gratings (HL)
When monochromatic light passes through a single slit of width a, it produces a central bright maximum flanked by dark minima at positions sinθ = mλ/a (m = ±1, ±2,...). A narrower slit produces a wider central maximum. In a double slit experiment, the double slit interference pattern (close, evenly spaced fringes) is modulated by the single slit diffraction envelope (wide, bright central region), causing some interference maxima to be suppressed where they coincide with diffraction minima. A diffraction grating has many closely spaced slits separated by d, producing very sharp maxima where nλ = d sinθ. Multiple slits produce sharper, brighter maxima than two slits alone, making gratings ideal for precise wavelength measurement.
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Frequently Asked Questions, IB Physics Wave Phenomena
What is Wave Phenomena in IB Physics? ↓
Reflection, refraction, diffraction and superposition for SL. HL adds single-slit diffraction patterns, double-slit modulation and diffraction gratings. This topic is part of Theme C (Wave Behaviour) in the current IB Physics syllabus.
Is Wave Phenomena SL or HL in IB Physics? ↓
Wave Phenomena is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Wave Phenomena? ↓
The key equations for Wave Phenomena are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Wave Phenomena? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Wave Phenomena for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.