# Homework Help: Determine the focal length of a lens

1. Feb 26, 2009

### rayhan619

1. The problem statement, all variables and given/known data

A 2.0-cm-diameter spider is 2.0 {\rm m} from a wall.
Determine the focal length of a lens that will make a half-size image of the spider on the wall.
Determine the position (measured from the wall) of a lens that will make a half-size image of the spider on the wall.

2. Relevant equations

M = fi/fo = di/do
1/f = 1/do + 1/di
3. The attempt at a solution
i got the first part. but not sure how to do the second one

2. Feb 26, 2009

### Delphi51

Re: lens

How did you get the first part? What did you get? There appear to be an infinite number of solutions, so I'm wondering which one you chose.

3. Feb 26, 2009

### rayhan619

Re: lens

that was in the book. im not really sure how to get that.
i tried it but i got -2m as the focal lens, where it is supposed to be 0.44m
here is how i did it

M = hi/ho
0.5 = hi/0.02
hi = 0.01 m

hi/ho = -di/do
0.01/0.02m = -di/2m
di = -1m

1/f = 1/do+1/di
1/f = 1/2m+1/-1m
f = -2 m

im not sure where i messed up

4. Feb 26, 2009

### Delphi51

Re: lens

You chose Do = 2 m.

Oh, I missed that - the image is on the wall 2m from the spider! So the lens must be between the wall and the spider. And Do + Di = 2. Between the two of us, we'll get this yet!
You can't choose Do = 2; you must put Do = 2 - Di.
Probably easiest to put it into 1/2 = Di/Do so you can solve for Di.
Then it is easy to get f. I get a little less than half a meter.
Kind of backwards, having to do (b) before (a).

5. Feb 26, 2009

Re: lens

got it
thanx a lot