Understanding light
Reflection, Fresnel & TIR
Light never just passes through a surface. Some of it always bounces — and sometimes all of it does.
The law of reflection
A mirror sends light away at the same angle it arrived, measured from the normal: θin = θout. Photonica reproduces this exactly; it is one of the validation checks, and it passes to the last digit.
Fresnel: the 4% you always lose
At every boundary between two materials, part of the light reflects even when it could pass straight through. For glass in air at normal incidence:
The fraction climbs steeply as the angle gets shallower, which is why a window looks transparent face-on and mirror-like at a glancing angle. Photonica computes the full Fresnel equations, separately for the two polarisation directions, at every surface.
Brewster's angle
At one specific angle, light polarised in the plane of incidence is not reflected at all:
Total internal reflection
Going the other way — from glass out into air — there is an angle beyond which light cannot escape at all. It reflects back inside, perfectly, with no loss:
Total internal reflection is what carries signals down an optical fibre, what makes the inside of a diamond flash, and what lets a prism turn a beam through 90° more efficiently than any mirror.
Coatings
Thin films on a surface change these numbers deliberately, by making reflections from the film's two sides cancel or reinforce. Photonica ships several and lets you stack your own:
| Coating | What it does |
|---|---|
| MgF₂ AR (550 nm) | The classic single-layer anti-reflection coat: drops 4% to about 1.3% |
| Broadband AR (QHQ) | A three-layer stack that keeps reflection low across the visible range |
| Dielectric mirror (HR 550) | Many layers making a near-perfect mirror at one wavelength |
| Dichroic (HR 650) | Reflects red, passes the rest — the beam splitter inside a projector |
Select a part and the Build panel shows the resulting reflectance per surface at 450, 550 and 650 nm, so you can see a coating work.