# Focal length of lens

1. May 1, 2010

### sp1974

The homework problem is "An object 26.0 cm in front of a certain lens is imaged 1.60 cm in front of that lens (on the same side as the object). What is the focal length (in centimeters) of this lens"

1/f = 1/di + 1/do

I tried solving based on having distance of object (do) and distance of image (di) so 1/26 + 1/1.6 = 1/f and I solved for f coming up with 1.51 cm. I also tried with -1.6 since they are on the same side coming up with -1.71 and the computer doesn't accept either

2. May 2, 2010

### Staff: Mentor

The image distance is negative (in the usual sign convention), so that is the correct approach.

3. May 2, 2010

### sp1974

so if I am understanding 1/26 + 1/-1.6 = 1/f and I should come up with -1.705 cm? It looks right but thinking the computer is being picky like my other answer

4. May 2, 2010

### Redbelly98

Staff Emeritus
5. May 2, 2010

### sp1974

yeah its picky. I have put in 1.7, 1.71 1.705 and nothing. It does it everyonce in a while. I show teacher and all will be good. Thanks!

6. May 2, 2010

### Redbelly98

Staff Emeritus
You might also try -1.70, that is the answer (-1.7049... cm) rounded to 3 sig figs.

Good luck!