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Seeing the Unseen : Rutherford’s Experiment

rutherfordboardTime Required:
30 minutes

Objective:

Measure the shape of a hidden object by analyzing entry and rebound paths for a marble rolled at the object.

Advance Preparation:

Plastic-foam shapes

Cut geometric shapes from a sheet of 1-inch plastic foam, such as a triangle, circle, half circle, rectangle, square, or are a challenge for students. More complicated shapes can be used in a second trial. Make the largest dimension of the shapes approximately 8 in.

Place a shape under a cover board at each lab station before the students come into the lab.

Cover boards

The cover boards must be sturdy enough not to bend when students write on them. Masonite or 1/4-inch plywood is adequate.

PURPOSE:
To discover how the physical properties, such as size and shape, of an object can be measured by indirect means.

BACKGROUND

Early in the 20th century scientists realized that all matter is made up of atoms. As a result of an experiment carried out by his assistants, Hans Geiger and Ernest Marsden, Ernest Rutherford suggested a model for the atom.

The experiment is known as Rutherford’s alpha particle scattering experiment. In it, a beam of alpha particles is aimed at a thin foil of gold. In the experiment:

Because of the results of this experiment, Rutherford showed that:

As you have done experiments, you have learned to make useful observations and to draw reasonable conclusions from data. But imagine how little you would be able to accomplish if the room in which you worked were so dark that you could not see the materials you were working with. Imagine how limited your observations would be if the object of your scrutiny were so small that it could not be seen, even with a microscope. When you think of how difficult experimentation would be under such adverse conditions, you will gain some appreciation for the enormous technical problems confronting early atomic scientists. These scientists had as their target the atom—a bit of matter so small that there was no hope of seeing it directly. Nevertheless, these ingenious experimenters were able to infer that the atom had a nucleus. It is impractical to reproduce the classic experiments that led to the discovery of the nucleus in a high school laboratory. You can get some idea of the challenge that these researchers faced, however, by playing the game described in this experiment. You will infer the size and shape of an object you cannot see or touch.


MATERIALS (PER TEAM)

To purchase a materials kit, visit Ward’s Science.


TEACHER NOTES:

Review with students how to relate each marble’s angle of reflection to its angle of incidence. Meter or yard sticks can be used to retrieve “lost” marbles without posing the associated hazard of revealing the mystery shape.

STUDENT PROCEDURE:

  1. At your lab station, you will find a sheet of cardboard resting on top of a hidden object. Do not look under the cardboard! Roll a marble under the cardboard from various directions and observe where it comes out. (Have your teacher retrieve the marble if it stays under the board; no peeking!)
  2. Place a sheet of construction paper on top of the board and trace the entry and exit path for each roll of the marble.
  3. Continue rolling the marble and recording its path until you think you know the size and shape of the object. Draw a full-sized sketch of the object on a sheet of paper. Check your results with your teacher. Do not look under the board until your teacher confirms your results.
  4. Ask your teacher for a second mystery object if you have time to repeat the game.

ANALYSES AND CONCLUSIONS

  1. How does this game simulate early efforts to determine the structure of the atom? In what ways is it different?
  2. You eventually had the satisfaction of seeing the shape under the board. Did the early atomic scientists have this same opportunity? Do scientists today have this opportunity?

GOING FURTHER

Develop a Hypothesis

On the basis of the results in this lab, develop a hypothesis about how the size, shape, or identity of other kinds of objects could be determined by indirect means.

 

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