# Optics question: What two positions of a lens gives a sharp image?

1. Aug 22, 2008

### LeeLoo

1. A lamp and a screen are 1m apart and a +4.5 D thin lens is mounted between them. Where must the lens be placed in order to produce a sharp image on the screen, and what will be the magnification?

2. Gauss Law: L' = L + F

3. I attempted this question by using the Gauss Law but I just don't know how to find the two possible positions of the lens to give a sharp image. I thought that a sharp image might be formed at F or F' but it doesn't work. Can someone please show me how to do this problem and explain it :)

2. Aug 22, 2008

### CompuChip

Why doesn't it work? What do you know about "F" ?

3. Aug 22, 2008

### tiny-tim

Welcome to PF!

Hi LeeLoo! Welcome to PF!

I don't think that's the right law …

shouldn't it have 1/… ?

4. Aug 22, 2008

### LeeLoo

Oh, yeah, the law is actually n/l + F = n'/l' but i thought people could abbreviate it like that. anyway, yay, i know how to do the question now!

1/l' + 1/l = F
l' +l =1m
l' = 1 - l

so sub that in and get two answers for 'l' which are the lens positions :D

and magnification= l'/l so you get m = -2 or -1/2

COOL!