Labs from Chicago, Summer 1993:
Calibrating the Sun's Light.

This is a Teacher's guide.

Objectives of lab:

• Establish a way to compare light intensities from the Sun and a "home-made" light source.
• Measure the luminosity of the portable light source as a function of solar luminosity. (This assumes they will know the distance to the Sun.)
• Begin to attack the problem of filters and measuring only a certain band of the Sun's spectrum.

Necessary tools:

• Sun (If it's a cloudy day, we'll have to set up an ersatz Sun in the lab.) Only one needed.
• Flashlights(with fresh batteries), heavy aluminum foil, tape and lengths of fiber optics. Will probably also need scissors, if foil isn't pre-cut. We'll need enough materials for one device per two students.
• Meter sticks, projection surface and things to cast shadows.
• Filters: The regular V filter for astronomical investigations & other miscellaneous ones. It would be great if I could get a large sheet of one (about a foot or more square).

Procedure:

1. Introduce students to what we'll be doing and build a bridge with what they've learned before. Have them first try using their flashlights alone on the screen described below.
2. Build fiber-optic light sources. This will be done by cutting the foil into pie shapes that can be rolled into cones. The fiber will be taped at the tip of the cone. The base of the cone will fit over the business end of the flashlight.
3. Try them inside with a setup that consists of a light bulb with its light directed on a sheet of paper. We'll need some sort of baffle to give a nearby shadow. Illuminating the shadow with their fiber optic source, each team measures the distance between the fiber and the shadow portion of the projection screen.
4. Go outside (this could also mean working on the balcony outside of the physics lounge on the second floor) and use the fiber- optic light sources to illuminate shadows. Measure the distance between the light source and the fiber optic.
5. Inside compute the intensity of the fiber source relative to the Sun using the inverse-square law.
6. Briefly discuss the results and try to understand the variables as well as the consistency and accuracy of this method.
7. Go out one more time and repeat the experiment using the V filter.
8. Discuss final results and christen the source we'll use at Yerkes.
9. Conclude by bridging to Yerkes activities. At some point in this activity it will be important to explain why we are using a fiber "point" source and not just the flashlight itself.

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