The angle between a line, r, and a plane, π, is the angle between r and its orthogonal projection onto π, r'.

That is, the angle between a line and a plane is equal to the complementary acute angle that forms between the direction vector of the line and the normal vector of the plane.

Let's name θ to the angle which line makes with plane,** u=(u**_{1},u_{2},u_{3}) the direction vector of line and **n=(A,B,C) **the normal vector of the plane.