Question

You are holding a convex lens at arm's length and looking at a distant street-light. You observe the up-side down image of the street light to be on your side of the lens and 60.0 mm from the lens. You then train the lens on your study lamp. If your study lamp is 90.0 mm from the lens, how far will the image of the lamp be from the lens (in mm)?

Answer 1

i'm assuming that your distant is very very far away.
from the lens eq,
\(\frac{1}{f}=\frac{1}{u}+\frac{1}{v}\)
since \(u >>v \), \(\frac{1}{v} >>\frac{1}{u} \)
the eq becomes
\(\frac{1}{f}=\frac{1}{v}\), f=60mm
for the next,
\(\frac{1}{f}=\frac{1}{u}+\frac{1}{v}\), where f=60mm, u=90mm
sub it all in to get v.
do you need further help?