CBSE 2025, Set 2

Question

Describe briefly Geiger-Marsden scattering experiment. Depict the graph showing the variation of number of scattered particles detected with the scattering angle. How did this graph lead to the discovery of the nucleus ?

Edvaya · Accepted Answer

The correct answer is Subjective. **Geiger-Marsden Scattering Experiment:**
In this experiment, a narrow beam of high-energy alpha ($\alpha$) particles from a radioactive source (like Bismuth) is collimated by lead bricks and directed to fall on a very thin gold foil (about $10^{-7}	ext{ m}$ thick). The scattered $\alpha$-particles are observed through a rotatable detector consisting of a zinc sulphide screen and a microscope, which produces flashes of light (scintillations) when struck by $\alpha$-particles.

![Geiger-Marsden Setup](https://www.edvaya.com/images/cbse/physics/2025/set2/q25-geiger-marsden-experiment-answer.webp)

**Graph:**

![Scattering Angle Graph](https://www.edvaya.com/images/cbse/physics/2025/set2/q25-scattering-angle-graph-answer.webp)

**Discovery of the Nucleus:**
The graph reveals that while most $\alpha$-particles pass straight through (indicating most of the atom is empty space), a small fraction of the incident $\alpha$-particles rebound back (deflect by an angle greater than $90^\circ$, approx 1 in 8000).

Because $\alpha$-particles are heavy and positively charged, such a massive head-on collision deflection could only occur if the entire positive charge and almost the entire mass of the atom are concentrated in a very small, dense, central volume. Rutherford named this tiny central core the **nucleus**.

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