Wavefronts, Rays, and Huygens' Principle
A wavefront is a surface of constant phase. Huygens' principle states that every point on a wavefront acts as a source of secondary wavelets; the forward envelope after time t is the new wavefront. In an isotropic medium, rays are normal to wavefronts.
Why this shows up in the exam
Designing lenses and mirrors · Tracing refraction without individual rays · Understanding diffraction at apertures
Learn the idea
A wavefront joins points of equal phase, and rays point normal to it. Picture ripples: every point on one crest is at the same stage of oscillation. Tiny secondary ripples from that crest build the next crest, so obstacles and media can reshape the front.
🧠 Memory hook: Fronts show phase; normals show travel.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- r perpendicular to wavefront — ray direction in an isotropic medium
- v = f lambda — wave speed relates frequency and wavelength
- Delta r = v Delta t — secondary-wavelet radius after time Delta t
How to approach it
- 1Identify the source or optical element
- 2Sketch the equal-phase surface
- 3Draw propagation normal to the front
Common slip-ups that cost marks
- •Calling a ray a wavefront
- •Drawing spherical fronts for a collimated beam
- •Changing frequency at a stationary interface
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A real object is placed 30 cm from a converging lens of focal length 10 cm. Find the real image distance.
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