Ans. Impact parameter is the perpendicular distance of velocity vector of $\alpha $ -particle from the centre of the nucleus.
If the impact parameter (b)is small, the angle of scattering $ (θ)$ is high.
For large impact parameter, the $\alpha $ -particle goes un-deviated.i.e,$θ=0$
For head-on collision, b is minimum and $\alpha $-particle rebounds back i.e, $θ=π$