球体的圆弧和阴影部分

Question

我想复制这张图片

但目前陷入困境：

• 圆弧（确保它们在半径与圆周相交处相交）
• 球体端盖的灰色阴影

以下是我目前所掌握的信息：

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\begin{document}
  \begin{tikzpicture}
  \draw (0,0) circle (2cm);
  \draw[dashed] (0,0) -- (2cm,0);
  \draw (0,0) -- node[midway, anchor=south east] {$R$} ++(60:2cm);
  \draw (0,0) -- ++(-60:2cm);
  \draw[dashed] (0,0) ++(60:2cm) arc (20:-20:5.1);
  \node at (0.35cm,0.25cm) {$\theta$};
  \end{tikzpicture}
\end{document}

虽然虚线弧是一个临时解决方案，但还是很接近，而且我不知道如何在端盖上添加阴影...

Answer 1

这是一个解决方案。

它使用椭圆弧代替圆弧，因为这样看起来更真实。但也有圆弧的代码，只是注释掉了。

另外，它还可以为背景添加阴影，使其看起来更好。

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\begin{document}
  \begin{tikzpicture}
  \draw (0,0) circle (2cm);
  \draw[dashed] (0,0) -- (2cm,0);
  \draw (0,0) -- node[midway, anchor=south east] {$R$} ++(60:2cm);
  \draw (0,0) -- ++(-60:2cm);

  % correct bounding box, the arc for the section foreground adds
  % some invisible stuff, which extends it
  \useasboundingbox (current bounding box);

  % with circular arcs
  %\draw[fill=gray,fill opacity=0.25] (2,0) ++(-120:2cm) arc (20:-20:-5.065) arc (60:-60:2cm);
  %\draw[densely dashed,fill=gray,fill opacity=0.25] (0,0) ++(60:2cm) arc (20:-20:5.065) arc (-60:60:2cm);
  % don't fill invisible part
  %\draw[densely dashed] (0,0) ++(60:2cm) arc (20:-20:5.065);

  % with elliptical arcs
  \draw[fill=gray,fill opacity=0.25]
      (0,0) ++(60:2cm)                  % start point
      arc (90:-90:-0.3 and {2*sin(60)}) % arc for section, foreground
      arc (-60:60:2cm);                 % arc for main circle
  \draw[densely dashed,fill=gray,fill opacity=0.25]
      (0,0) ++(60:2cm)                  % start point
      arc (90:-90:0.3 and {2*sin(60)})  % arc for section, background
      arc (-60:60:2cm);                 % arc for main circle
  % don't fill background part
  %\draw[densely dashed] (0,0) ++(60:2cm) arc (90:-90:0.3 and {2*sin(60)});

  \node at (0.35cm,0.25cm) {$\theta$};
  \end{tikzpicture}
\end{document}