The reflective property of parabolas The figure here shows a typical point
\(P\left(x_{0}, y_{0}\right)\) on the parabola \(y^{2}=4 p x .\) The line \(L\) is
tangent to the parabola at \(P .\) The parabola's focus lies at \(F(p, 0) .\) The
ray \(L^{\prime}\) extending from \(P\) to the right is parallel to the \(x\) -axis.
We show that light from \(F\) to \(P\) will be reflected out along \(L^{\prime}\) by
showing that \(\beta\) equals \(\alpha\) . Establish this equality by taking the
following steps.
a. Show that \(\tan \beta=2 p / y_{0}\) .
b. Show that \(\tan \phi=y_{0} /\left(x_{0}-p\right)\)
c. Use the identity
$$\tan \alpha=\frac{\tan \phi-\tan \beta}{1+\tan \phi \tan \beta}$$
to show that \(\tan \alpha=2 p / y_{0}\)
since \(\alpha\) and \(\beta\) are both acute, \(\tan \beta=\tan \alpha\) implies
\(\beta=\alpha\)