Q.
A point object $O$ can move along vertical line $A B$ as shown in the figure. At $t=0$, the image of the object is visible to $D$ and then the object $O$ is released from rest from point $A$. The time for which object is visible to $D$ is $T$. Find $T^{2}$. (Given $L=9.8 \,m$ )
Ray Optics and Optical Instruments
Solution:
Required time is given by
$3 L=\frac{1}{2} g t^{2} \Rightarrow t=\sqrt{\frac{6 L}{g}}$
