Classroom Activity: How Far? How Small?

Main Idea: Students will use a simple set up to measure the change in the apparent size of an object as it moves away from them.

Learning Objectives:
Time: 30 to 45 minutes

Materials: Each 2-person team will need:
Skills Needed:
Preparation: Each team should construct a viewer to improve the consistency of their measurements.
Ruler showing hole in card
1. Measure and cut out a square with 1/2 inch sides from near the middle of one end of the index card.

Folded card

2. Fold the index card lengthwise along the bottom of the square hole, then open the fold to a right angle.

Taping card to card-board strip

3. Tape the folded card to the end of the cardboard strip aligning the hole with the right edge of the strip. Make sure the tape does not block the hole.


1. Measure the actual height of the object to the nearest 1/4 inch and record it in decimal form.

2. Stretch out a tape measure across a table so that 0 inches is even with the near edge of the table. Use tape to secure the tape measure in place.

3. Place the viewer at right angles to the table edge and aligned with the tape measure. Make the index card stick out exactly 10 inches. Secure the viewer with tape.

4. Sit or kneel on the floor and put one eye next to the hole in the viewer.

5. Hold or prop the ruler vertically, 0 end down, at the edge of the table so you can read it through the viewer.

6. Have your partner move the object 10 inches away along the tape measure. Look through the viewer and note where the top edge of the object now appears to be on the ruler. Using the Data Sheet, record this as the apparent height in decimal form.

7. Move the object away to 20, 30, 40, 50 and 60 inches. View and record the apparent height at each location.

8. Switch roles with your partner to collect a second set of data.

Analyzing the Results:
  1. Make a graph of distance away versus apparent height.
    • Use your graph to make a prediction about the object's apparent height at a distance you did not test. Then check your prediction by actually making a measurement at that distance.
    • Make a prediction from your graph about how far away your object would need to be to look one inch tall. If your table is long enough, test this prediction as well.
  2. Divide the actual height by the apparent height for each distance. The result is a ratio you can use for comparison with other teams who used different sized objects.

Drawing Conclusions:

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