Question

A ray of light parallel to principal axis after reflection from concave mirror passes through ___________.

Answer 1

A ray of light parllel to principal axis (shows that u=$\infty$) after reflection from concave mirror (of focal length f) passes through.
$\frac{-1}{u}+\frac{1}{v}=\frac{1}{f}$
So $\frac{-1}{\infty }+\frac{1}{v}=\frac{-1}{f}$
$\frac{1}{v}=\frac{-1}{f}$
$v=-f$
i.e. it passes through focus.