# Draw a point on a curve or surface; also draw a line in 3D plot?

I would like to draw a point on a curve or on a surface. For example, I would like to plot the point $(1,1,\sqrt{2})$ with a special color.

Plot3D[{Sqrt[4 - x^2 - y^2]}, {x, 0, 2}, {y, 0, 2},
PlotStyle -> Blue, AxesLabel -> Automatic]


Or the point $(1,\sqrt{3})$ on the curve

Plot[{Sqrt[4 - x^2]}, {x, 0, 2}, PlotStyle -> Blue]


I know how to plot a plane in a 3D plot, but I don't know how to plot a straight line in such a plot. How is it done?

• Show[ Plot3D[Sqrt[4 - x^2 - y^2], {x, 0, 2}, {y, 0, 2}, PlotStyle -> Blue, AspectRatio -> 1], Graphics3D[{Red, Sphere[{1, 1, Sqrt@2}, .1]}] – Dr. belisarius Sep 9 '15 at 17:57
• Plot[Sqrt[4 - x^2], {x, 0, 2}, PlotStyle -> Red, Epilog -> {Blue, PointSize[Large], Point[{1, Sqrt@3}]}] – Dr. belisarius Sep 9 '15 at 18:00

Show[Plot3D[{Sqrt[4 - x^2 - y^2]}, {x, 0, 2}, {y, 0, 2}, PlotStyle -> Blue, AxesLabel -> Automatic], Graphics3D[{Red, PointSize[.05] , Point[{1, 1, Sqrt@2}]}]]
Sphere[{1, 1, Sqrt@2}, .2]

Show[Plot[{Sqrt[4 - x^2]}, {x, 0, 2}, PlotStyle -> Blue],Graphics[{Red, PointSize[.05] , Point[{1, Sqrt@3}]}]]