Lens positions to focus a 2.0 cm candle flame on a wall 2.1 m away

[surfahgirl]

Homework Statement

A 2.0--tall candle flame is 2.1 m from a wall. You happen to have a lens with a focal length of 37 cm .

How many places can you put the lens to form a well-focused image of the candle flame on the wall?

For each location, what are the height and orientation of the image?

Homework Equations

1/f=1/s'+1/s

h'=(s'/s)h

The Attempt at a Solution

So I think there are 2 places you can place the lens because s'+s=210cm and s' and s are interchangeable or something like that. I'm not quite sure though.

But then for the second part, i found the heights to be .65 and 6.2 cm but either one or both are wrong. I know I have to find the values for s and s' and I'm not quite sure how to do that. I thought s' would just be 2.1 but how can you have two values for it then?

 

[rl.bhat]

Put s' = 210 - s and substitute it in the lens formula to find s.

 

[surfahgirl]

ah..just realized that myself.
that's so simple. i was definitely over thinking it.

thanks!