Under what conditions a diagonal matrix is orthogonal

Question

I found in this answer :

Identity Matrix = Orthogonal?

That the diagonal should have an absolute value $= 1$

But I don't see exactly why.. Thanks in advance for your help

Answer 1

You want $A^TA=I$. If $A$ is diagonal, then the $i$th diagonal entry of $A^TA$ is $a_{ii}^2$ and must equal $1$