Second focal point of a concave lens

[Fiona Rozario]

Why is the focal point to the left of a concave lens called the second focal point? Isn't it just a matter of the name? So can't the focal point on the left of the lens be called F1 instead of F2?

 

[adjacent]

Why not? Focal point is just a name.It's not even real in concave lenses.

 

[jtbell]

When you have incoming parallel rays, the outgoing rays converge towards or diverge from the first focal point.

When you have outgoing parallel rays, the incoming rays converge towards or diverge from the second focal point.