Answer
$36^{\circ}$
Work Step by Step
We can find the required angle as follows:
$\lambda=\frac{v}{f}$
$\implies \lambda=\frac{340}{1300}=0.2615m$
We know that
$W sin\theta=m\lambda$
This can be rearranged as:
$\theta=sin^{-1}(\frac{m\lambda}{W})$
We plug in the known values to obtain:
$\theta=sin^{-1}[\frac{(1)(0.2615m)}{0.84m}]$
$\theta=18^{\circ}$
Now the angle between the first two minima is given as
$\alpha=2\theta$
$\implies \alpha=2(18^{\circ})$
$\alpha=36^{\circ}$
