# Need help with focal length lens lab

1. Apr 24, 2004

### MandolinGirl

In my physics lab, we have been assigned to find the focal length of a lense that was given to us. The professor says it can be found by using this formula: Object distance over 1 plus 1 over the Image Distance will equal one over the Focal Length.

So, we put a piece of white paper at ground level, then got a flashlight and measured its diameter and determined it to be 2 1/8. During our first attempt, the flashlight was held at a distance of 24” from the paper. The lens was held at a distance of 20” inches from the paper, and 4” from the light source. Then the amount of magnification was measured by measuring the light as it was projected through the lens and onto the paper. In the second attempt, we kept the light source at a constant distance of 24” from the paper, but changed the position of the lens by holding it 6” from the light source and 18” from the paper. We then measured the magnification of the light beam again and planned to use it for comparative purposes. In the case where the lense was held 4" from the light source, the magnification was 5 1/4 inches, and in the case where the lense was held 6" from the light source, the magnification was determined to be 4 1/2. I just don't understand how to use those numbers in a formula that will ultimately determine the focal length. Thank you for any help you can provide!

Sarah

2. Apr 25, 2004

### UrbanXrisis

I think it's 1/do+1/di=1/f

I remember it as "If I do I die" ----> 1/f=1/do+1/di