A good answer might be:

Slopes of Perpendicular Lines

You may recall this from past math classes:

If two lines are perpendicular, then the product of their slopes is -1.

This is another way to look at what goes on when you make a normal vector to a given 2D vector by swapping elements and negating one:

If  v   =   ( x, y )T
Then  v'   =  ( -y, x )T   is orthogonal,
because   ( x, y)T · ( -y, x )   is -xy + yx   =   0.
The slope of   ( x, y )T   is y/x.
The slope of   ( -y, x )T   is -x/y.
The product of the slopes is   y/x * -x/y   =   -(xy)/(xy)   =   -1


Are ( -1.5, 6)T and (2, 2)T orthogonal?