Since the focal length is the same on both sides, we draw a second ray from the top of the candle, through the F prime focal point, which then refracts through the lens and travels parallel to the axis.

由于两侧的焦距相同， 我们从蜡烛顶部绘制第二条光线， 通过 F 主焦点，然后通过透镜折射并平行于轴传播。