In the α-particle scattering experiment, all the α-particles of the beam have nearly the same kinetic energy. The shape of the trajectory of a scattered α-particle depends on which of the following?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Only on the impact parameter
An α-particle is turned aside by the Coulomb repulsion of the gold nucleus. The impact parameter b is the perpendicular distance of the initial velocity vector of the α-particle from the centre of the nucleus.
Option (1) Only on the impact parameter — Correct. With the kinetic energy the same for every α-particle, b alone decides how close the particle comes to the nucleus. A small impact parameter gives a large scattering. In a head-on collision (minimum b) the particle rebounds. A large b leaves it nearly undeviated.
Option (2) On the source of the α-particles — Incorrect. The source only supplies the beam. All the α-particles leave it with nearly the same kinetic energy. So it cannot make one particle bend more than another.
Option (4) On the α-particle and the screen material of the detector — Incorrect. All the α-particles are alike. The zinc sulphide screen only shows, by a flash of light, where a scattered particle arrives. The path is already fixed by the encounter with the nucleus in the foil.
NCERT (2026-27):
Alpha-particle Trajectory – “The trajectory traced by an α-particle depends on the impact parameter, b of collision. … (In a beam, all particles have nearly same kinetic energy.)”