Step 1: The amplitude (or argument) of a complex number \( z = a + bi \) is given by:
\[
\text{Amplitude of } z = \arg(z) = \tan^{-1} \left( \frac{b}{a} \right).
\]
Step 2: For \( \frac{1 + \sqrt{3}i}{\sqrt{3} + 1} \), we calculate the argument using the formula. The result is \( \frac{\pi}{6} \).