7. Let the right vertex of the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1(a>b>0)$ be $A$. If there exists a point $P$ on the ellipse such that $\angle O P A$ $=\frac{\pi}{2}$ ( $O$ is the origin), then the range of $\frac{b}{a}$ is
7. $\left(0, \frac{\sqrt{2}}{2}\right)$