# Combinations of lenses

1. Apr 14, 2009

### synergix

1. The problem statement, all variables and given/known data
As shown in the diagram below (not to scale), a 3.0 cm high object is placed 50.0 cm from thin converging lens A that has a focal length of 10.0 cm. Converging lens B with a focal length of 15.0 cm is placed 25.0 cm to the right of lens A. Find and describe the final image.
http://webct6.nic.bc.ca/webct/RelativeResourceManager/Template/CourseMaterials/CourseContent_2007FA/Assignments/PHY060W_Assignment_09_files/image013.jpg [Broken]

2. Relevant equations

1/di = 1/f - 1/do

3. The attempt at a solution

1/ dia = 5/50.0cm - 1/50.0cm= 1/12.5 cm

dia=12.5cm

dob= 25.0cm - 12.5 cm = 12.5 cm

fb= 15.0cm

1/dib= 1/15.0cm- 1/12.5cm=

12.5/187.5 - 15.0/187.5=

-2.5/ 187.5

dib= -75

so now to my question. I am not sure what a negative distance indicates when dealing with a combination of lenses. does it mean the image is virtual? because I know from my diagram that it is on the opposite isde of the lenses from where the light is coming.

Last edited by a moderator: May 4, 2017