Why are some things darker when wet? • Hacker News Board

Post by amluto » Sun, Jan 12, 2020, 10:14 PM UTC

sjg007 wrote: post #153

amluto wrote: post #150

Earlier quoted context omitted.

Metals and, generally, homogenous materials (dielectrics, conductors, etc). Light reflects off of a conductor or a dielectric due to a relatively straightforward solution to Maxwell’s equations. In contrast, anything phosphorescent most definitely absorbs and re-emits, sometimes hours later. Imagine glow-in-the-dark pigment.

Thanks, yes makes sense. But do metals actually reflect or do they just absorb and just release spontaneously?

This probably depends on your perspective. If you think of a metal as a homogenous linear medium (which is a good approximation for long wavelengths but is not exact), then light is a wave, photos are an unnecessary detail, and the wave mostly reflects and is absorbed as heat. If you look the metal as a whole bunch of atoms and a sea of mobile, discrete electrons, then the incoming photons excite the sea of electrons and, when the interaction settles down, photons are leaving. If you insist on modeling the electrons with QED, congratulations, you’ve made your life unnecessarily complicated unless the photons have such high energies that electrons are created or end up with relativistic velocities. If you believe in string theory, maybe the photons were only an approximation in the first place.

Physics is all about choosing an appropriate model of the world that captures the detail you need without being more complicated than needed. Is a person on a playground swing a pendulum, or is it a bag of molecules sitting on a flexible piece of rubber suspended by a bunch of rusty metal links from a flexible steel bar that is, in turn, supported by many geological layers in the Earth’s crust?