In which of the following cases, the value of impact parameter (for alpha particle scattering experiment by Geiger-Marsden) will be minimum ?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → head on collision of \( \alpha \)-particle with nucleus
The impact parameter b is the perpendicular distance of the initial velocity of the \( \alpha \)-particle from the centre of the nucleus. A particle aimed straight at the nucleus has \( b = 0 \), the smallest possible value, and it is turned back through nearly \( 180^\circ \).
The other cases correspond to larger values of b: a particle that goes undeviated passes far from the nucleus (large b), small scattering also goes with a large b, and large scattering goes with a small b but not the smallest, which is the head-on case.
NCERT (2026-27):
Alpha-Particle Trajectory – “… The impact parameter is the perpendicular distance of the initial velocity vector of the α-particle from the centre of the nucleus (Fig. 12.4). … It is seen that an α-particle close to the nucleus (small impact parameter) suffers large scattering. In case of head-on collision, the impact parameter is minimum and the α-particle rebounds back (θ ≅ π). For a large impact parameter, the α-particle goes nearly undeviated and has a small deflection (θ ≅ 0).”
