Answer
$40^{\circ}$
Work Step by Step
We can find the required angles as follows:
$I=I_{\circ}cos^2\theta cos^2(80-\theta)$
We know that the possible values of $\theta$ are $10^{\circ}, 20^{\circ}, 30^{\circ}, 40^{\circ}$
But for $\theta=40^{\circ}$, we obtain the maximum of $I_{\circ}$.
$\implies I=I_{\circ}cos^2(40^{\circ}).cos^2(40^{\circ})$
$I=(0.344)I_{\circ}$
Thus, the required angle is $40^{\circ}$.
