Focal Length of Lens: Solving for f with Object and Image Distances

[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

 

[Doc Al]

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

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

 

[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

 

[Redbelly98]

Looks good to me, though there may be an issue with how many significant figures you report in your answer.

FYI, a cool feature of wolframalpha is solving problems like this:
http://www.wolframalpha.com/input/?...ble-&a=UnitClash_*f.*DegreesFahrenheit.dflt--

 

[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!

 

[Redbelly98]

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

Good luck!