About

Illustrated Physics is a growing collection of illustrated essays. Each one takes a single idea and draws it until the argument is visible.

What this is not

It is not a textbook and not a reference. There is no syllabus, no problem sets, no obligation to cover anything in particular. An essay exists because there is a figure that explains something better than a paragraph can — and if there is no such figure, there is no essay.

The figures are computed, not drawn

Every illustration is generated as SVG at build time by code in this repository. None are photographs, none are exported from a drawing program, and none are bitmaps. The consequence that matters is not that they are sharp or small, though they are both:

• They are correct by construction. A ray refracting from air into glass bends by the angle Snell's law requires, because the angle is computed. A pendulum's phase portrait really is a level set of its energy.
• They are parameterised. The same generator draws the incline at 27° or at 5°, and the essay picks.
• They respond to the light and dark themes, because they reference colour roles rather than colours.

The full set of generators is listed on the figure library page.

Where the model stops

Physics is a stack of models, each excellent inside a range and wrong outside it, and the range is usually left implicit. This site tries to make it explicit: where a figure relies on an approximation, the essay says which one and shows the size of the error. The small-angle assumption behind "a pendulum's period does not depend on amplitude" is not a detail — it is the whole content of the claim, and it is worth a figure of its own.

On being wrong

Corrections are welcome and will be made. A figure that is beautiful and wrong is worse than no figure, because it is more persuasive.