8. A line passing through the focus $F$ of the parabola $y^{2}=2 p x$ intersects the parabola at points $A, B$. The projections of points $A, B$ on the directrix of this parabola are $A_{1}, B_{1}$, respectively. Find the size of $\angle A_{1} F B_{1}$.
8. According to the definition of a parabola, $|A F|=\left|A A_{1}\right|,|B F|=\left|B B_{1}\right|, \angle A A_{1} F=\angle A F A_{1}, \angle B B_{1} F=$ $\angle B F B_{1}$, since $\angle A_{1} A F+\angle A A_{1} F+\angle A F A_{1}+\angle B B_{1} F+\angle B F B_{1}+\angle B_{1} B F=360^{\circ}$, and $\angle A_{1} A F+$ $\angle B_{1} B F=180^{\circ}$. Therefore, $\angle A F A_{1}+\angle B F B_{1}=90^{\circ}$, hence $\angle A_{1} F B_{1}=90^{\circ}$.