
$\Delta x=\pi R$
Let $\lambda$ be the wavelength of the waves. Now for minima to occur at $\mathrm{D}$, path difference $=\left(n+\frac{1}{2}\right) \lambda$ where $n=0,1,2, \ldots \ldots \ldots$ Thus $\frac{(2 n+1) \lambda}{2}=\pi R \Longrightarrow \lambda=\frac{2 \pi R}{2 n+1}$
Now $n=0,1,$ and $2 \Longrightarrow \lambda=2 \pi R, \frac{2 \pi R}{3}$ and $\frac{2 \pi R}{5}$ respectively.

