light
Light begins as rays you can draw with a ruler, becomes a wave that interferes and diffracts, then a field that propagates itself through empty space — and finally a photon, countable and quantised. The same word carries four increasingly demanding pictures, each one absorbing the last.
Units touched optics · electromagnetism · quantum Adjacent threads waves · fields
The stations
This thread has no stations authored in Wave 1. Its derivations arrive once the optics and electromagnetism units are built out — the stops below are the planned route.
Rays, then wavefronts
Reflection and refraction from Fermat's principle of least time — where geometric optics is shown to be the short-wavelength limit of something larger.
Interference and diffraction
Two slits, one screen. Light must be a wave to build the fringe pattern — the first station where the ray picture visibly breaks.
Light as an electromagnetic wave
Maxwell's equations in vacuum yield a wave that travels at c. Light stops being a separate subject and becomes a consequence of the field.
The photon
The photoelectric effect and blackbody spectrum force energy quanta E = hν. Light is counted, not just measured — and the wave picture must now share the stage.
When authored, each station will link to a derivation in the vault and sit inside its unit. Compare how the same idea is handled elsewhere on the threads index.