\begin{tikzpicture}[scale=0.75]

% Constants

	% Sphere radius
	\def\r{2}

% Clip region

	\tkzInit[xmin=-0.5,xmax=4.5,ymin=-0.7,ymax=2.5]
	\tkzClip

% Points

	% Origin (viewpoint)
	\tkzPoint*(0,0){O}

	% Sphere center
	\tkzPoint*(4,0){C}

	% Intersections

		% Directionpoint for tangent ray
		\tkzPoint*(1,0.58){Odt} % Odt = Directionpoint at O for vector t

		% Intersect tangent ray with maximum sphere
		\tkzInterLCR(Odt,O)(C,\r cm){U}{I} % I = back facing intersection

	% Unit vectors

		% Calculate tangent ray vector
		\tkyUnitVect(O,Odt){t}

	% Create vectorpoints

		% Tangent ray at O
		\tkyPointVectL(O,t,0.8){Ovt} % Ovt = vectorpoint at O for vector t

% Drawing

	% Points

		% Origin
		\tkzDrawPoint[pos=below left](O)

		% Sphere center
		\tkzDrawPoint(C)

		% Tangent ray intersection point
		\tkzDrawPoint[pos=above left](I)

	% Line segments

		% Dashed ray line
		\tkzSegment[style=dashed](Ovt/I)

		% Segment OC of length d
		\tkzSegment[style=dashed,label=$d$,poslabel=-16pt,time=0.43](O/C)

		% Segment IC of length r
		\tkzSegment[label=$r$,style=dashed](I/C)

	% Vectors

		% Tangent ray
		\tkzDrawVector(O/Ovt);
		\tkzDrawPoint[name=\unit{t},pos=above left,size=0](Ovt)

	% Sphere
	
		\tkzCircleR[lw=1pt](C,\r)

	% Angles

		% Right angle at I
		\tkzRightAngle(O/I/C)

		% Tangent angle
		\tkzMarkAngle[size=0.5,dist=0.15,labelsize={},label={\tau}](I/O/C)

\end{tikzpicture}
