A light ray emerging from the point source placed at P(2,3) is reflected at a point Q on the y-axis. It then passes through the point R(5, l0). The coordinates of Q are 

The point of coincidence on $\text{ the }y\text{ -axis is }Q(0,\lambda )\text{ . }$

The image of P(2,3) on the $y\text{ -axis is }{P}_1(-2,3)$

Therefore, 

$\text{ Slope of }{P}_{1}Q=\text{ Slope of }{P}_{1}R$

$\text{ or }\frac{\lambda -3}{0-(-2)}=\frac{10-3}{5-(-2)}$

$\begin{array}{l}\text{ or }\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\lambda -3=2\\ \text{ or }\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\lambda =5\end{array}$

Therefore, the point Q is (0, 5).