Where the model stops
When two waves meet, they simply add
Waves pass through each other unchanged and their displacements add point by point. From that one impoverished-sounding rule comes interference, beats, and the evidence that light is a wave at all.
OpticsThe bend at the boundary, and what it is really about
Light changes direction when it changes speed. Snell's law is the geometry of that statement, and it can be derived without knowing anything about light at all.
OpticsThe angle past which light cannot leave
Snell's law asks for the sine of an angle greater than one, and no such angle exists. What happens instead is a perfect mirror made out of nothing but a change of speed.
ElectromagnetismField lines are a choice, not a discovery
Nothing in space is arranged in lines. The lines are a drawing convention — and an unusually good one, because three separate facts about the field survive the translation.
ElectromagnetismThe field before the lines were drawn on it
A field is a vector attached to every point of space. Drawing it as arrows on a grid is honest and ugly; drawing it as lines is beautiful and throws information away.
ThermodynamicsThe speeds in a still room
The air in a quiet room is not still. Every molecule in it is moving at hundreds of metres per second, and temperature is a single number summarising an entire distribution.
ThermodynamicsThe ceiling on every engine, set before it was designed
There is a maximum efficiency no heat engine can exceed, and it depends on nothing but two temperatures. Not the fuel, not the working substance, not the cleverness of the engineer.
RelativityTwo axes, one speed, and a diagram that does the arguing
Put position across and time up, insist that light travels at forty-five degrees for everyone, and nearly every result in special relativity becomes something to read off rather than derive.
RelativityNow is a choice of slicing
Two events happening at the same time is not a fact about the events. It is a fact about who is asking, and different observers slice spacetime at different angles.