In the adjacent diagram, $\mathrm{CP}$ represents wavefront and $\mathrm{AO}$ and $\mathrm{BP}$ corresponding two rays. One particular condition on $\mathrm{\theta }$ for destructive interference at $\mathrm{P}$ between the ray $\mathrm{BP}$ and reflected ray $\mathrm{OP}$ is $\mathrm{cos\theta }=\frac{\mathrm{\lambda }}{\mathrm{nd}}$, where $\mathrm{n}$ is