Rutherford's Experiment


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Rutherford's Alpha-ray Scattering Experiment

(Geiger and Marsden Experiment)

Pictorial representation of the a particles through different angles.

Geiger and Marsden Experiment

A radioactive source S emitting a particles was collimated into a fine beam and made to fall on a thin gold foil. The a particles scattered in all directions. These produced bright flashes on falling on the fluorescent screen. A detector was used to observe this. On plotting a graph between the angles of scattering q and number of alpha particles N, the following observations were noted:

angles of scattering q and number of alpha particles

alpha rays scattering in detector

(i) Most alpha particles pass straight through the gold foil or suffer very small angle of scattering.

(ii) Here few a particles scattered through large angles (greater than 90o)

alpha particles scattered through large angles

alpha rays scattering in narrow beam

(iii) An a particle is rarely scattered through 180o.

Explanation

These observations explained that there was a coulomb interaction between a particles and electron. As a particles were 7000 times more massive than electrons, a strong force is needed to deflect them through large angles. Therefore the entire +ve charge of the gold foil was concentrated at the center called the atomic nucleus. If a a particle had a head on collision with the nucleus it scattered through 180o. Thus it was seen that a particles, which passed at larger distance from the nucleus, go almost undeviated. The scattering being due to elastic collision between nucleus and a particles, it was observed that due to Coulomb's repulsive force the K.E. of the alpha particles kept on decreasing as it approached the gold nucleus.

alpha particle is rarely scattered through 180 degrees

At a certain distance ro from the nucleus, the K.E. of a particle was completely converted to electric potential energy and this prevented the a particle to go further closer and so it retraced its path. This distance was called the distance of closest approach.



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