# Homework Help: Converging lens (focal L)

1. Aug 4, 2009

### NikkiNik

1. The problem statement, all variables and given/known data
The figure illustrates qualitatively a converging lens focusing monochromatic rays from A to B. The vertical and horizontal scales of the figure are not the same. The top ray leaves A at an angle of 2.38 deg above the optic axis and leaves B at an angle of 1.25 deg below the optic axis. The total distance from A to B is 75.3 cm.

http://i32.photobucket.com/albums/d2/NikkiNik88/convLens3.gif

Calculate the focal length of the lens for that light.

2. Relevant equations

1/d0 + 1/di = 1/f

3. The attempt at a solution

I'm not even sure where to begin when I'm given the angles. I think 75.3 cm = d0 + di
but I'm not sure where to go from there

2. Aug 4, 2009

### tiny-tim

Hi NikkiNik!

This is a geometry exercise …

you have two triangles, you know the lengths a+b and h, and you know a/h and b/h.

3. Aug 4, 2009

### rl.bhat

If AOP is the axis of the lens, and AP and BP are the incident and refracted rays, then OP/do = tan 2.38 and BP /di = tan 1.25.
Or do*tan2.38 = di*tan1.25
And do + di is given.
Find do and di and hence focal length of the lens.