Question 149
Given a right triangular prism $A_1B_1C_1 - ABC$ with $\angle BAC = \frac{\pi}{2}$ and $AB = AC = AA_1 = 1$, let $G, E$ be the midpoints of $A_1B_1$, $CC_1$ respectively; and $D, F$ be variable points lying on segments $AC, AB$ (not including endpoints) respectively. If $GD \perp EF$, the range of the length of $DF$ is ( ).