Model of an ideal black body; a cavity with a hole (see Wikipedia).
Find other black body diagrams via the “black body” tag: black body color gradient, plots, Plank oscillators.
% Author: Izaak Neutelings (March 2019)% Inspiration: https://tex.stackexchange.com/questions/203597/automatically-generate-graphics-which-shows-light-diffusion-on-a-rough-surface\documentclass[border=3pt,tikz]{standalone}\usetikzlibrary{decorations.pathmorphing,decorations.markings,calc} % for random steps & snake\usetikzlibrary{arrows.meta} % for arrow size\tikzset{>=latex} % for LaTeX arrow head\tikzstyle{radiation}=[-{Latex[length=2,width=1.5]},red!95!black!50,opacity=0.7,very thin,decorate,decoration={snake,amplitude=0.7,segment length=2,post length=2}]\begin{document}% BLACK BODY\begin{tikzpicture}[scale=1,rotate=10]\shade[top color=black!60,bottom color=black!80,shading angle=10] % background(7:1) arc (7:355:1);\fill[thick,black,postaction=decorate, % rough inner surfacedecoration={markings,mark=between positions 0.55 and 1 step 0.03 with {\node[transform shape,inner sep=1pt](hit\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) {};}}](7:1) arc (7:353:1) --++ (-7:-0.18)decorate[decoration={random steps,segment length=2,amplitude=1pt}]{arc (-7:-353:0.82)} -- cycle;\draw[yellow] % connect light ray to random points(8:1.5) -- (hit6.center) -- (hit1.center) -- (hit15.center) -- (hit5.center) --(hit9.center) -- (hit14.center) -- (hit2.center) -- (hit10.center) -- (hit3.center) --(hit4.center) -- (hit11.center) -- (hit13.center);\foreach \ang in {-35,-5,35}{\draw[radiation] (1,0)++(\ang:0.1 and 0.2) --++ (\ang:0.35);}\end{tikzpicture}% BLACK BODY - without infalling light\begin{tikzpicture}[scale=1,rotate=10]\shade[top color=black!60,bottom color=black!80,shading angle=10] % background
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