Answer
$I$ reaches its maximum value as $\cos^2\theta$ reaches its maximum value, which is $1$.
At $\theta=0$, $\cos^2\theta=1$, so at $\theta=0$, $I$ reaches its maximum value.
Work Step by Step
$$I=k\cos^2\theta$$
From the formula of Lambert's Law, we see that $I$ would reach its maximum value when $k\cos^2\theta$ also reaches its maximum value.
However, since $k$ is a constant and as a result, does not change its value, the maximum value of $k\cos^2\theta$ happens at the maximum value of $\cos^2\theta$.
Overall, the maximum value of $I$ occurs when $\cos^2\theta$ reaches its maximum value.
We remember that the range of $\cos\theta$ is $[-1,1]$. In other words, $$-1\le\cos\theta\le1$$
That means, $$0\le\cos^2\theta\le1$$
Therefore, the maximum value of $\cos^2\theta$ is $1$.
We find that as $\theta=0$, $\cos^2\theta=1$, which is the maximum value of $\cos^2\theta$, meaning also the maximum value of $I$ has been reached.
Therefore, the maximum value of $I$ occurs when $\theta=0$.
