7 The position vectors of the points \(A\) and \(B\), relative to an origin \(O\), are given by
$$\overrightarrow { O A } = \left( \begin{array} { l }
1
0
2
\end{array} \right) \quad \text { and } \quad \overrightarrow { O B } = \left( \begin{array} { r }
k
- k
2 k
\end{array} \right)$$
where \(k\) is a constant.
- In the case where \(k = 2\), calculate angle \(A O B\).
- Find the values of \(k\) for which \(\overrightarrow { A B }\) is a unit vector.