#optics
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From script to sentence: a traceable Brewster-angle calculation
A tiny Fresnel calculation shows how this site now carries computed values from versioned inputs into prose without mistaking traceability for correctness.
No lens burns hotter than the sun: étendue and the second law
Give me a perfect lens of any size and I still cannot heat a target past about 5800 K with sunlight. The limit is not engineering, and it is not diffraction — it is Liouville's theorem wearing an optics costume, and it hands back the solar constant as a receipt.
Refraction is absorption you can't see
A material that absorbed no light at any frequency would have a refractive index of exactly 1 — no lenses, no prisms, no rainbows. Refraction is not the opposite of absorption; it is a causal side effect of it, and the Kramers–Kronig relations say exactly how much.