Indirect Observations and Inference

Introduction

Many of the experiments that led 20th century scientists to discover the nature of the atom involved indirect observation and inference. Lead students through this demonstration exercise in indirect observation, providing them with a deeper understanding of how scientists like Ernest Rutherford “knew without seeing.”

Background

During the early 1900s, scientists such as J.J. Thomson (1856–1940) and Ernest Rutherford (1871–1937) were faced with a very difficult challenge: to develop an accurate mental picture (or model) of the atom—an object much too small to be directly observed. Every year many students faithfully study schematic diagrams of Rutherford’s “Gold Foil Experiment” without ever experiencing the thinking that led those scientists to their revolutionary conclusions.

In the early 1900s, the prevailing theory for the make-up the atom was Thomson’s “plum pudding” model. From his earlier work, Thomson theorized the atom was a sphere of diffuse positive charge with electrons embedded in the sphere, not unlike raisons in a plum pudding. Rutherford’s “Gold Foil Experiment” was designed to help validate this model. An alpha particle emitter, radium, was place in front of the surface of thin gold foil. The gold foil and radium were surrounded by a circular sheet of zinc sulfide (ZnS) which was used as a detector: the ZnS sheet would light up when hit with alpha particles. By directing a beam of alpha particles normally onto the sheet of gold foil, the alpha particles should all have been deflected by, at most, a few degrees. Measuring the pattern of scattered particles was expected to provide information about the distribution of charge within the atom. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees, some even deflected straight back.

Rutherford took these data and deduced the atom was mostly empty space, with electrons moving around a very small, dense, positive core he called the nucleus, with the diameter of the atom itself more than 100,000 times greater than that of its nucleus.

In the following demonstration, students are asked to form a mental picture of an object hidden beneath a cardboard blind. Students must formulate hypotheses about the size, shape and composition of the object while watching you probe the object with a laser beam.

Materials

(for each demonstration)
Binder or paper clips, jumbo, 4
Blotting paper, 12" x 19", 6
Cardboard square, 17" x 17"*
Laser pointer*
Marker
Mirrors, Plexiglas, 2½" x 3½", 3*
Plastic strip, 3" x 88"*
Transparent tape
*Materials included in kit.

Safety Precautions

Please follow the safety guidelines described by the laser’s manufacturer. Never look directly at the laser light. For further safety information, please see The Safe Use of Lasers write-up. As always, follow all laboratory safety guidelines.

Prelab Preparation

Experiment Set Up

  1. Assemble the plastic ring. Take the strip and overlap the ends by 3 inches. Secure the overlap with four binder clips (see Figures 1 and 2).
    {12162_Preparation_Figure_1}
    {12162_Preparation_Figure_2}
  2. Obtain the six sheets of blotting paper and lay them out in a two by three grid (see Figure 3). Using transparent tape, connect the sheets together as shown in Figure 4.
    {12162_Preparation_Figure_3}
    {12162_Preparation_Figure_4}
  3. Tape the three mirrors together to form an equilateral triangle.
  4. Flip the blotting paper assembly over. Using a meter stick, measure the diagonal of the grid and mark the center point with a marker (see Figure 5). Create an outline of the cardboard top by placing the top in the center of the blotting paper and tracing its outline onto the blotting paper (see Figure 6).
    {12162_Preparation_Figure_5}
    {12162_Preparation_Figure_6}
    {12162_Preparation_Figure_7}
  5. Center the plastic ring on the blotting paper and place the mirror assemble at the center with one side parallel to one edge of the blotting paper assembly (see Figure 7). Place the cardboard square on top of the mirrors in line with the marked square of the paper.

Procedure

{12162_Procedure_Figure_8}
  1. Begin by presenting students with the fundamental problem faced by early 20th-century scientists who were seeking a model (or mental picture) of the atom: “How do you develop a mental picture of something that you cannot see?” Explain that, although you cannot see the atom, a mental picture can be developed though indirect observation and measurement.
  2. Distribute the worksheet to the students. Have the students gather around the demonstration table.
  3. Explain the demonstration to the students. “You are about to participate in an exercise of indirect observation.” Have the students read the directions for the exercise on their worksheets.
  4. Position the laser pointer at the upper left-hand corner of the cardboard blind and fire the beam to the other side (see Figure 8). Have students mark on their worksheets the location of the red dot on the plastic ring.
  5. Move the laser down approximately 2 cm along the edge of the cardboard square. Fire the laser again. Have students mark the location of the red dot.
  6. Repeat the process until all four sides of the object have been explored.

Student Worksheet PDF

12162_Student.pdf

Teacher Tips

  • This kit contains enough materials to do the demonstration many times over.
  • If there are too many students in class to gather comfortably around a table, the demonstration can be shown on a screen using a flexible camera.
  • This demo can act as an introductory analogy for teaching the specifics of Rutherford’s Gold Foil Experiment.
    The triangle mirror is analogous to the gold atom in the thin foil.
    The laser pointer is analogous to the lead box containing the alpha particle emitter.
    The beam of laser light is analogous to the alpha particle stream.

Answers to Questions

Observations and Worksheet

The laser will be “fired” every few centimeters along the length of all four sides of the cardboard square. Below are four blank diagrams of the experiment. For each side examined, draw a ray for the initial and final points of each beam of laser light. See examples.

{12162_Answers_Figure_9}
{12162_Answers_Figure_10}
{12162_Answers_Figure_11}
{12162_Answers_Figure_12}

Post-Lab and Analysis
  1. Based on your indirect observations and collected data, what inferences can you make about the object underneath the cardboard?

    What is its shape and dimensions?

    The object is an equilateral triangle approximately 3.5 inches on a side.

    What is its surface like?

    Since light is reflected when it strikes the surface and does not penetrate, the surface must be mirrored.

  2. Recall Rutherford’s gold foil experiment from your textbook.

    How is this demonstration analogous to Rutherford’s classic experimental set-up?

    The triangle mirror is analogous to the gold atom in the thin foil.
    The laser pointer is analogous to the lead box containing the alpha particle emitter.
    The beam of laser light is analogous to the alpha particle stream.
    The plastic ring is analogous to the zinc sulfide circular detector.

References

Special thanks to Jonathan McClintock, St. Albert High School, Council Bluff, IA, for providing the idea and the instructions for this activity to Flinn Scientific.

Next Generation Science Standards and NGSS are registered trademarks of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.