If $\frac { z - \alpha } { z + \alpha } ( \alpha \in R )$ is a purely imaginary number and $| z | = 2$, then a value of $\alpha$ is : (1) 1 (2) $\frac { 1 } { 2 }$ (3) $\sqrt { 2 }$ (4) 2
If $\frac { z - \alpha } { z + \alpha } ( \alpha \in R )$ is a purely imaginary number and $| z | = 2$, then a value of $\alpha$ is :\\
(1) 1\\
(2) $\frac { 1 } { 2 }$\\
(3) $\sqrt { 2 }$\\
(4) 2