Chapter 38 - Photons and Matter Waves - Problems - Page 1184: 65a

$nn^*$ is a positive real number.

$n = a + ib$ We can find an expression for $nn^*$: $nn^* = (a + ib)(a - ib)$ $nn^* = a^2+b^2$ Since $a$ and $b$ are positive or negative real numbers, $a^2+b^2$ is a positive real number. Therefore, $nn^*$ is a positive real number.