Let a vertical tower AB   of height 2h  stands on a horizontal ground. Let from a point P  on the ground a man can see upto height h   of the tower with an angle of elevation $2\alpha$ . When from  P  he moves a distance  d in the direction of $\overrightarrow{AP}$,  he can see the top B of the tower with an angle of elevation $\alpha$  . If $d=\sqrt{7}h$  , then  $\tan \alpha$ is equal to