# Projection lens problem

1. Feb 7, 2005

### nemzy

The projection lens in a certain slide projector is a single thin lens. A slide 20.0 mm high is to be projected so that its image fills a screen 2.0 m high. The slide-to-screen distance is 3.00 m.
(a) Determine the focal length of the projection lens.

(b) How far from the slide should the lens of the projector be placed in order to form the image on the screen?

This is how i attempted to solve the problem

(a) p=3m
M=h'/h = -q/p

so... h'/h = -q/p

solve for -q,

then plug in the q value for 1/p + 1/q = 1/f and solve for f

*is q suppose to be positve or negative, i am not sure how the sign conventions works

anyways, i get the wrong answer...anyone know whawt i did wrong?

also i have no idea how to do part B

2. Feb 8, 2005

### clive

It seems that $$p+q=3 m$$ and not $$p=3 m$$, Try with to find p and q from
$$\frac{p}{q}=\frac{2 m}{20 mm}=100$$
and
$$p+q=3$$
and then compute f with
$$\frac{1}{p}+\frac{1}{q}=\frac{1}{f}$$.

OBS. I think you use the physical sign-convention, then all the variables in my post are positive.
p-distance from the screen to the lens;
q- distance from the lens to the slide.