Two beams of monochromatic light with intensities $64 \mathrm{mW}$ and $4 \mathrm{mW}$ interfere constructively to produce an intensity of $100 \mathrm{mW}$. If one of the beams is shifted by an angle $\theta ,$ the intensity is reduced to $84 \mathrm{mW}$. The magnitude of $\theta$ is