Question

A beam of fast moving alpha particles were directed towards a thin film of gold. The parts A, B and C of the transmitted and reflected beams corresponding to the incident parts A, B and C of the beam are shown in the adjoining diagram. The number of alpha particles in

• A. B' will be minimum and in C' maximum
• B. A' will be maximum and in B' minimum
• C. A' will be minimum and in B' maximum
• D. C' will be minimum and in B' maximum

Answer 1

The Correct Option is B

In the case of alpha particles directed towards a thin film of gold, the scattering of the particles occurs due to the interference between the transmitted and reflected beams.
This scattering is a typical phenomenon observed in diffraction patterns, where the intensity of the scattered particles varies as a function of the angle.
When the incident beam passes through the thin film, interference results in maxima and minima for the transmitted and reflected parts of the beam.
\( A' \), which corresponds to the transmitted part, will experience maximum intensity as it passes through the region of the film where constructive interference is most prominent.
\( C' \), corresponding to a reflected part, will experience a minimum intensity because destructive interference occurs at this point.

The correct answer is (B) : A' will be maximum and in B' minimum.

Answer 2

This is an example of the Rutherford scattering experiment, where alpha particles are directed towards a thin gold foil. The transmitted and reflected beams correspond to the alpha particles that either pass through the gold foil or are scattered.

1. Incident beam parts: 
  - Part A of the beam corresponds to the straight-through or transmitted alpha particles, which pass through the gold foil without much scattering.
  - Part B of the beam corresponds to the alpha particles that are scattered at small angles.
  - Part C of the beam corresponds to the alpha particles that are scattered at large angles.

2. Transmitted and reflected beams:
  - The part \( A' \) (transmitted beam) will have the maximum number of alpha particles since most particles pass through the foil without much deflection.
  - The part \( C' \) (reflected or highly scattered beam) will have the minimum number of alpha particles since very few alpha particles are scattered at large angles.
  - The part \( B' \) corresponds to the moderate scattering angles and will have a number of particles that lies between the other two.

Thus, the number of alpha particles in \( A' \) will be maximum, and in \( C' \) it will be minimum, as the majority of particles pass straight through with minimal scattering.