The half-unit in one line
What light does at a surface, at a lens, and at an obstacle small enough to make it behave like the wave it is. Waves and optics is worth 8 of Physics's 50 questions over six chapters; this map covers the three optical ones, and the other three have their own map at Sound.
The chapters
| Chapter | What it covers | Read |
|---|---|---|
| Reflection, refraction and dispersion | Curved mirrors, refraction at plane surfaces and through lenses, total internal reflection, dispersion, chromatic aberration and achromatism | 19 min |
| Interference | Coherence and the conditions for interference, Young's double-slit experiment, and where interference is put to work | 13 min |
| Diffraction and polarisation | Single-slit diffraction and the grating, resolving power of optical instruments, polarisation and Brewster's law | 14 min |
About three quarters of an hour in all. The optical three are longer than the wave three, and the first of them is longer than the other two together — budget two sittings rather than one.
Read in this order
Read reflection, refraction and dispersion first. It is the longest of the three and it fixes the sign convention that both formulae in the chapter — and every image question in the paper — depend on. Nothing later in optics makes sense while the signs are still guesswork.
Then read interference before diffraction and polarisation. Diffraction is superposition from a continuous slit rather than two, and the grating equation is the double-slit condition generalised; a student who meets the grating first has to learn path difference twice.
One cross-link is worth making: wave motion, in the sound half, defines wavelength, phase and superposition in their simplest setting. If waves are new to you, read that chapter before interference and the physical-optics pair will cost half the effort.
The two halves are examined as one unit and can be revised as one, but they do not have to be read together. Optics is the half that rewards practice over rereading, so leave time for the chapter tests rather than spending it all on the notes.
How it is asked
MEC sets the paper roughly half recall, a third understanding and a fifth application. Recall here is a definition or a named law: Brewster's angle, what a grating does, which aberration a doublet corrects. Understanding is why the central fringe is bright, why a convex mirror always gives a diminished virtual image, or why sound cannot be polarised but light can. Application is one substitution — an image distance, a fringe width, a grating order, a critical angle — where the only real risk is a sign or a unit.
Geometrical optics carries most of this half's questions, because mirrors, lenses and total internal reflection give an examiner more ways to set a numerical than a grating does. Physical optics is where the conceptual questions live.
Then practise
Each chapter has a fixed-length chapter test under Practice › Topics, drawn from questions written to that chapter's scope. The mirror and lens questions are worth re-sitting until the signs are automatic.