An Improvement to the Inverse Square Law Light Experiment
Richard Borne (Northern Illinois University) has suggested an improvement to the Inverse Square Law Light Experiment using our Optics Expansion Kit (order code OEK). He noticed that the intensity of our white LED lamp decreases somewhat over time when it is first turned on. Since students doing this experiment may take a few minutes to collect the light intensity vs. distance data, his suggestion is to turn on the LED light sources for a half hour or more before starting data collection. We have modified the documentation in our Optics Expansion Kit and the instructions for our Inverse Square Law lab to include these suggestions. Dr. Borne has nicely documented light intensity variation and its impact on the experimental results in the article.
The Vernier white LED light source assembly shows a significant decrease in illumination with time as the light source assembly warms up:
During the time that it might take for a typical student to perform the inverse square law experiment, say 15 to 20 minutes actually collecting data, this reduction in illumination over time would be expected to increase the value of the exponent obtained for the inverse “square” law:
The blue line on the above Excel chart shows the results when the experiment is performed by starting with the lamp at room temperature. The red line shows the results when the experiment is performed by starting with the lamp warmed up for about an hour. An hour was selected since the illumination of the LED decreases relatively slowly after an hour of warm up time. Without LED warm-up the inverse “square” law provided a power of 2.236. With an hour of warm-up time the inverse “square” law provided a power of 1.965 ? much closer to the desired value of 2.
Clearly, students should be advised of the need to have the Vernier white source LED properly warmed up in order to obtain the best possible inverse square law results.