What you will learn in Wave Optics

1. 1. Every Point Is a New Source

   Ray optics has no explanation for any of its own rules. Huygens' construction is where the explanations come from, and it starts by treating a wavefront as a crowd of sources rather than a thing that travels.

2. 2. The Rules Optics Could Only Assert

   Snell's law, drawn rather than applied. The construction and the rule agree, and the construction knows nothing about the rule — which is what makes it an explanation rather than a restatement.

3. 3. Turning a Wavelength into a Millimetre

   Young's experiment before it became an argument about photons: a way of measuring a length no instrument of 1801 could reach, by converting it into one a ruler can.

4. 4. The Slit That Refuses to Cast a Shadow

   A single opening produces a pattern of dark bands, and narrowing it spreads the light further rather than less. Both facts come out of adding up what a continuous row of sources sends to each angle.

5. 5. Where the Ray Came From

   Ray optics is not a separate theory that stops applying somewhere. It is this same calculation at small λ/a, and the whole of the Optics module sits at one end of a single slider.

6. 6. Three Filters, and the Sieve That Isn't

   Adding a third polariser between two crossed ones lets light through. Nothing that merely removed the wrongly-oriented light could do that, and the result is what forces a better picture of what a polariser does.

7. 7. The Angle That Removes Glare

   At one angle a surface reflects none of the light oscillating in the plane of incidence, so the reflection is completely polarised. It is why polarising sunglasses work, and the reason is geometric.