A simple experiment to measure the inverse square law of light in daylight conditions

The measurement of the irradiance of a point source of light as a function of the distance to the source is a nice and simple experiment for introductory physics courses. The resulting 1/r² power law appears in other contexts and can be easily understood by the conservation of the total energy traversing concentric spherical surfaces of different radii. The typical verification of this law in the laboratory involves the use of a darkened room, a big restriction for this important experiment to be implemented in a modern inquiry based curriculum. To avoid this restriction, we designed a simple apparatus to measure the falloff of the irradiance with distance that can be used in daylight conditions.