Question

When a butterfly lands 12 cm from a concave mirror, it's image is 18 cm behind the mirror.
Find the focal length
1/f=(1/do)-(1/di)
1/f=(1/12)-(1/18)
1/f=0.0833-0.0555
1/f=0.0277
f=36

I'm pretty confident w/ my math but as I understood, it's do-di if it's behind the mirror, which it is, & f is also negative if it's behind the mirror. So my question is, is it not always negative? or do I make 36 negative to show that it's in the other direction?

Thanks a bunch :)

Answer 1

If you are using this convention:
https://apps.spokane.edu/InternetContent/AutoWebs/AsaB/Phys103/MirrorsThinLens.pdf

then your calculations are correct
di=-18
do=+12

1/12-1/18=1/36

=> f=36 cm

Answer 2

Ok thank you :)

Answer 3

You're welcome!