# Concave Lens Ray Tracing

1. Feb 9, 2010

### chrisfnet

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

I have a concave lens where f < do < R. I have to provide a ray tracing diagram.

3. The attempt at a solution

I drew the focal ray through the focus and parallel to the principal axis. I drew the principal ray parallel to the principal axis then through the focus. I drew a dashed line from the center past the object tip and to the lens.

This leads to a diagram where there are no converging lines. However, if I arbitrarily choose some numbers to confirm this... this is where I get confused.

I'll choose a focal length (f) of 5cm. This means that R must be 2f. So, R = 10cm.

I'll choose an so between 5cm and 10cm of 7.5cm.

Given that:

1/so + 1/si = 1/f

1/0.075m + 1/si = 1/0.05m

si = 0.15m

M = -0.15m/0.075m = -2

This is telling me that I have a real image that is inverted and magnified by a factor of two.

Confused. Any help would be appreciated.

2. Feb 9, 2010

### Stonebridge

Does your diargam look anything like this?

http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter2_files/image008.gif [Broken]

Last edited by a moderator: May 4, 2017