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Let I explain it as , To draw a perpendicular to the plane surface is easy.Reason is very simple taking plane surface as a straight line and draw another line inclined to it at right angle.So you have two mutually perpendicular lines.Any line in plane surface is perpendicular to this line.So you have set of mutually perpendicular lines at pont of incidence in the plane surface.In case of curved surface we do not have such liberty . So to construct a set of mutually perpendicular lines ,we draw a tangent at the ponit of incidence at curved surface.At this point ,on this line we can draw perpendicular ,which we call normal to the surface at the point.
To find direction of normal at point of incidence ,we draw tangent.
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