Refraction, Diffraction and Interference
Applying refractive index, total internal reflection, diffraction, coherence, path difference and interference to optical and other wave systems.
How to study A-level Physics
Define the system, represent the physics, select a justified model, calculate transparently, then test the result against units, limits and evidence.
Core concepts
Concept 1
Refraction changes wave speed and wavelength at a boundary while frequency remains fixed.
Exam cue: Draw the normal and identify the media before applying a refraction law.
Concept 2
Diffraction is most pronounced when aperture or obstacle scale is comparable with wavelength.
Exam cue: State the coherence condition and source geometry in interference explanations.
Concept 3
Stable interference requires coherent waves, with maxima and minima determined by path or phase difference.
Exam cue: Relate fringe spacing to wavelength, geometry and separation only within the model assumptions.
Risk pitfalls and guardrails
Saying frequency changes when a wave enters another medium.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Confusing diffraction with refraction.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Using the critical-angle condition for light travelling from lower to higher refractive index.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Memory anchors
Refraction
Refraction is a direction change caused by a change in wave speed at a boundary.
Refractive Index
Refractive index compares light speed in vacuum with speed in the medium.
Critical Angle
At the critical angle, the refracted ray travels along the boundary from the higher-index side.
Diffraction
Diffraction is strongest when gap or obstacle size is comparable with wavelength.
Coherence
Coherent sources maintain a constant phase difference.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
Light travels at 2.00 × 10⁸ m s⁻¹ in a material. Using c = 3.00 × 10⁸ m s⁻¹, what is its refractive index?
When light enters glass from air, which quantity remains unchanged?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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