Gray circles (See master page.)*
White paper punchouts from a three
hole paper punch*
White paper
Double convex lenses of different
diameters
Metric ruler
Astronomical telescope (optional)
*per student
Procedure:
1. Have students examine several different double convex lenses.
2. Compare the ability of each lens to gather light by focusing the light from overhead fixtures onto a piece of white paper. Which lens produces a brighter image? Be sure to hold the lenses parallel to the paper.
3. Compare the light gathering power of five imaginary lenses (gray circles) by placing small white paper circles (punchouts) on each. The number of punchouts represent the number of photons collected at a moment of time. Students may draw their own circles with compasses for this step.
4. What is the mathematical relationship between the number of punchouts that a circle can hold and the circle's diameter? How did you arrive at this conclusion?
Discussion:
In a dark room, the pupil of the eye gets bigger to collect more of the dim light. In bright sunlight the pupil gets smaller so that too much light is not let into the eye. A telescope is a device that effectively makes the pupil as large as the objective lens or mirror.
A telescope with a larger objective lens (front lens) or objective mirror collects and concentrates more light than a telescope with a smaller lens or mirror. Therefore, the larger telescope has a greater light gathering power than the smaller one. The mathematical relationship that expresses light gathering power (LGP) follows:
