a tangent
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They are the angles made by the incident ray and the reflected ray with the line perpendicular to the reflecting surface at the point of reflection.
Would be at at 90 degrees to the surface, in other words at right angles. So a line which was perpendicular to a horizontal surface would be vertical.
It's called the normal.
'two perpendicular number lines are called'Special:Search
Perpendicular bisector.