I would like to draw a mirror and its thickness, using Tikz. I'll show you the final result I'd like to achieve

I managed to write this code (using tikz package)

-- (75:3.5) arc (75:-75:3.5)
-- (-75:3) arc (-75:75:3);

which gives back this picture

I wish to have a different shading effect, like a ring one. Can someone help me please? Thank you in advance.

ADDENDUM: I'm gettin really mad. I tried to swap inner color with outer color to obtain this effect

but this is what actually latex gives me back

I'll leave you the code I used

\begin{scope}[xshift=6cm]
\clip (75:-3)
-- (75:-2.5) arc (75:-75:-2.5)
-- (-75:-3) arc (-75:75:-3);
\end{scope}

If I keep outer and inner colors like the case before, I obtain this

I'm not understading why tikz is acting like this. Theoretically if I swap the colors, I should get what I am supposed to get, but I'm clearly wrong and I don't know why.

Welcome! These are two basic options from the pgfmanual:

• circle with fuzzy edge 15 percent

Example:

\documentclass[tikz,border=3mm]{standalone}
\begin{document}
\begin{tikzpicture}
\begin{scope}
\clip (75:3)
-- (75:3.5) arc (75:-75:3.5)
-- (-75:3) arc (-75:75:3);
\end{scope}
\begin{scope}[xshift=5cm]
\clip (75:3)
-- (75:3.5) arc (75:-75:3.5)
-- (-75:3) arc (-75:75:3);
\end{scope}
\end{tikzpicture}
\end{document}

You can use this e.g. as follows:

\documentclass[tikz,border=3mm]{standalone}
\usepackage{contour}
\contourlength{0.05em}
\tikzset{->-/.style={decoration={% https://tex.stackexchange.com/a/39282/194703
markings,
mark=at position #1 with {\arrow{>}}},postaction={decorate}},
->-/.default=0.5}
\begin{document}
\begin{tikzpicture}[>=stealth,every label/.append style={black},
bullet/.style={circle,fill=black,inner sep=0.1em}]
\begin{scope}
\clip (75:3)
-- (75:3.5) arc (75:-75:3.5)
-- (-75:3) arc (-75:75:3);
\end{scope}
\draw (-3,0) coordinate (L) -- (4,0) coordinate (R) (3,0)
node[bullet,label=below right:\contour{white}{$V$}](V){};
\pgfmathsetmacro{\myangle}{atan2(3*sin(30),2+3*cos(30))}
\draw[thick,orange,->-] (-2,0) node[bullet,label=below:$O$](O){}
-- (30:3) node[bullet,label={above right:\contour{white}{$P$}}] (P){};
\draw[dashed,blue] (0,0) node[bullet,label=below:$C$] (C){} -- (P);
\path[overlay] (P) -- ++ (180+60-\myangle:1) coordinate (Q);
\draw[thick,red,->-] (P) --
(intersection cs:first line={(L)--(R)}, second line={(P)--(Q)})
node[bullet,label=below:$I$] (I){};
\path[blue]
pic [draw, "$\scriptstyle\alpha$",angle eccentricity=1.25,angle radius=1cm] {angle=R--O--P}
pic [draw, "$\scriptstyle\beta$",angle eccentricity=0.7,angle radius=0.8cm] {angle=R--C--P}
pic [draw, "$\scriptstyle\gamma$",angle eccentricity=0.7,angle radius=0.8cm] {angle=R--I--P};
\end{tikzpicture}
\end{document}

\documentclass[tikz,border=3mm]{standalone}
\usepackage{contour}
\contourlength{0.05em}
color(0pt)=(pgftransparent!100); color(22.5bp)=(pgftransparent!80);
color(25bp)=(pgftransparent!20); color(50bp)=(pgftransparent!00)}%

\tikzset{->-/.style={decoration={% https://tex.stackexchange.com/a/39282/194703
markings,
mark=at position #1 with {\arrow{>}}},postaction={decorate}},
->-/.default=0.5}
\begin{document}
\begin{tikzpicture}[>=stealth,every label/.append style={black},
bullet/.style={circle,fill=black,inner sep=0.1em}]
\begin{scope}
\clip (75:3)
-- (75:3.5) arc (75:-75:3.5)
-- (-75:3) arc (-75:75:3);
\end{scope}
\draw (-3,0) coordinate (L) -- (4,0) coordinate (R) (3,0)
node[bullet,label=below right:\contour{white}{$V$}](V){};
\pgfmathsetmacro{\myangle}{atan2(3*sin(30),2+3*cos(30))}
\draw[thick,orange,->-] (-2,0) node[bullet,label=below:$O$](O){}
-- (30:3) node[bullet,label={above right:\contour{white}{$P$}}] (P){};
\draw[dashed,blue] (0,0) node[bullet,label=below:$C$] (C){} -- (P);
\path[overlay] (P) -- ++ (180+60-\myangle:1) coordinate (Q);
\draw[thick,red,->-] (P) --
(intersection cs:first line={(L)--(R)}, second line={(P)--(Q)})
node[bullet,label=below:$I$] (I){};
\path[blue]
pic [draw, "$\scriptstyle\alpha$",angle eccentricity=1.25,angle radius=1cm] {angle=R--O--P}
pic [draw, "$\scriptstyle\beta$",angle eccentricity=0.7,angle radius=0.8cm] {angle=R--C--P}
pic [draw, "$\scriptstyle\gamma$",angle eccentricity=0.7,angle radius=0.8cm] {angle=R--I--P};
\end{tikzpicture}
\end{document}

ADDENDUM: As for the addendum to your question: here is a shading that produces something of that sort. It was obtained by looking up the definition of circle with fuzzy edge 10 percent and to "invert it", as you suggest.

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{decorations.markings,calc,angles,quotes}
color(0pt)=(pgftransparent!100); color(22.5bp)=(pgftransparent!100);
color(25bp)=(pgftransparent!00); color(50bp)=(pgftransparent!00)}%
\tikzset{->-/.style={decoration={% https://tex.stackexchange.com/a/39282/194703
markings,
mark=at position #1 with {\arrow{>}}},postaction={decorate}},
->-/.default=0.5}
\begin{document}
\begin{tikzpicture}[>=stealth,every label/.append style={black},
bullet/.style={circle,fill=black,inner sep=0.1em}]
\begin{scope}[xshift=6cm]
\clip (75:-3)
-- (75:-2.5) arc (75:-75:-2.5)
-- (-75:-3) arc (-75:75:-3);
\end{scope}
\draw (-2,0) coordinate (L) -- (7,0) coordinate (R)
(3,0)  node[bullet,label=below left:$V$](V){}
(6,0)  node[bullet,label=below:$C$](C){}
(-1,0)  node[bullet,label=below:$O$](O){}
($(C)+(160:3)$) node[bullet,label=above:$P$](P){};
\draw[orange,->-,thick] (O) -- (P);
\draw[red,->-,thick] let \p1=($(P)-(C)$),\p2=($(P)-(O)$),
\n1={180-atan2(\y1,\x1)},\n2={atan2(\y2,\x2)} in
(P) -- ++({180-(2*\n1+\n2)}:2);
\draw[dashed, shorten >=-2cm] (C) -- (P);
\end{tikzpicture}
\end{document}

• thank you, much appreciated – frad Dec 16 '19 at 22:15
• @user203430 Remember to mark this answer as accepted if you think it adequately answered your question! – sintax Dec 17 '19 at 19:16
• is there any way to get the shading like the one obtained with the first method? Why if I swap inner with outer color, I got that result? – frad Dec 18 '19 at 11:03
• @user203430 Please note that fade out fades out over the full radius. However, we clip only a tiny shell of the circle, along which the color does not vary very much. If you remove the \clip ....; you will see what I mean. – user194703 Dec 18 '19 at 15:25