7. In $\triangle A B C$, the sides opposite to angles $A, B, C$ are $a, b, c$ respectively. If the sizes of angles $A, B, C$ form a geometric sequence, and $b^{2}-a^{2}=a c$, then the radian measure of angle $B$ is $\qquad$
$$
\begin{array}{l}
\text { Since } \sin ^{2} B-\sin ^{2} A=(\sin B+\sin A)(\sin B-\sin A) \\
=2 \sin \frac{B+A}{2} \cos \frac{B-A}{2} \cdot 2 \cos \frac{B+A}{2} \sin \frac{B-A}{2}=\sin (B+A) \sin (B-A) \\
\text { then } b^{2}-a^{2}=a c \Rightarrow \sin (B+A) \sin (B-A)=\sin A \sin C \Rightarrow \sin (B-A)=\sin A \\
\Rightarrow B-A=A \text { or } B-A=\pi-A(\text { discard }) \Rightarrow B=2 A .
\end{array}
$$
$$
\text { Therefore } A+B+C=7 A=\pi \Rightarrow A=\frac{\pi}{7} \Rightarrow B=\frac{2 \pi}{7} \text {. }
$$