### The Inverse Square Law

Posted:

**Thu Mar 04, 2010 3:36 pm**We found your great website when we were searching for a way to build a photometer. My project is comparing light intensities of two different types of light bulbs. I've completed my science fair testing and I've finished my data tables and graphs. I am wanting to use the inverse square law to compare the light intensities of incandescent light bulbs and compact fluorescent lamps (CFLs), but I am having trouble with the formula. I need to now how the formula goes from Light Intensity 1 / Distance 1 squared = Light Intensity 2 / Distance 2 squared to Light intensity 2 = Light intensity 1 * Distance 2 squared/Distance 1 squared. What happens to the "Light intensity 1" in the last formula when trying to solve for Light intensity 2? Do you throw it out or do you call it one? And, why?

I plugged in the data I got, which was the distance each light source was from the photometer. For one of my tests, the Incandescent light bulb was 85.5 cm from photometer. The CFL was 77.5 cm from photometer. After using the inverse square law formula, I got that the incandescent light bulb's light intensity was 1.25 times greater than the CFL's. Is this right? If a judge asks me what I did with the "light intensity 2" from the formula, I won't know how to answer the question.

I got the inverse square law formula(s) from your website.

Thank you for you help!

I plugged in the data I got, which was the distance each light source was from the photometer. For one of my tests, the Incandescent light bulb was 85.5 cm from photometer. The CFL was 77.5 cm from photometer. After using the inverse square law formula, I got that the incandescent light bulb's light intensity was 1.25 times greater than the CFL's. Is this right? If a judge asks me what I did with the "light intensity 2" from the formula, I won't know how to answer the question.

I got the inverse square law formula(s) from your website.

Thank you for you help!