Planet Earth
International Astronomical Union WGSN

Preliminary Experiment: Distance Law

Before we can measure planets, we work out the basic law of photometry: as the distance from the lamp increases, the detector receives less and less light.

Set up

Tape Measure

Measure the irradiance every 10 cm at distances from the lamp: ten readings between 10 and 110 cm

photograph of the set up, evaulation (by ninth graders)

Result: The decline is quadratic.

Sketch of the quadratic law: The initial energy is distributed over growing spherical shells of area 4 pi r².