# Checking if Line and Plane Intersect

The post is compiled with information on Wikipedia page on Line-plane Interaction and this notes on finding the normal to a plane.

## What is the question?

This post is about checking if a line intersect with a plane, given 2 different coordinates on the line and 3 different coordinates on the plane.

## How to solve it?

Suppose having a line with coordinates \(\mathbf{a}\) and \(\mathbf{b}\) on the line, the vector equation representing the line with the set of \(\mathbf{p}\) consist the line is: \begin{equation} \label{eq:line} \mathbf{p} = \mathbf{a} + d \mathbf{(b - a)} \end{equation}

Suppose having a plane with coordinates \(\mathbf{e}\), \(\mathbf{f}\) and \(\mathbf{g}\) on the plane, then the normal to the plane is \(\mathbf{n = (f - e) \times (g - e)}\). If \(\mathbf{p_0}\) is a point on the plane (e.g. \(\mathbf{p_0}\) can be \(\mathbf{e}\), \(\mathbf{f}\) or \(\mathbf{g}\)), the plane can be expressed as the set of points \(\mathbf{p}\) for which: \begin{equation} \label{eq:plane} \mathbf{(p - p_0) \cdot n} = 0 \end{equation}

The point(s) where the line and the plane intersect, the points have the same coordinates. Substitute \eqref{eq:line} into \eqref{eq:plane}:

\[\begin{align*} ((\mathbf{a} + d \mathbf{(b - a)}) - \mathbf{p_0}) \cdot \mathbf{n} &= 0\\ d \mathbf{(b - a)} \cdot \mathbf{n} + \mathbf{(a - p_0)} \cdot \mathbf{n} &= 0\\ d &= \frac{\mathbf{(p_0 - a)} \cdot \mathbf{n}}{\mathbf{(b - a)} \cdot \mathbf{n}} \end{align*}\]If \(\mathbf{(b - a)} \cdot \mathbf{n} = 0\) then the line and the plane are parallel. In this case, if \(\mathbf{(p_0 - a)} \cdot \mathbf{n} = 0\) then the line is on the plane. Otherwise the line and plane have no intersection.

If \(\mathbf{(b - a)} \cdot \mathbf{n} \neq 0\) there is a single point of intersection.