Determining the focal length of a compound lens

  • #1
41
2

Homework Statement


A biconvex lens, [itex]L_1[/itex], with focal length of magnitude [itex] \left|
f_1
\right|
= 15[/itex] cm is used with a second biconvex lens, [itex]L_2[/itex], with focal length of magnitude [itex] \left|
f_2
\right|
= 25[/itex] cm to form a compound lens system.

If the two lenses are placed in mutual contact what is the focal length of the compound lens?

Homework Equations



[itex]\frac{1}{f_c} = \frac{1}{f_1} + \frac{1}{f_2}[/itex]

The Attempt at a Solution


It's just a matter of plugging into the above equation.. However do I use the the absolute values or the fact that for a biconvex lens

[itex]
f_1 > 0[/itex]

and

[itex]
f_2 < 0[/itex]?

Solution given is [itex] +9.375 [/itex] cm, which suggests you use the absolute values, however I am not fully convinced as it was not specified in the course notes whether to use absolute values or not.

Not using absolute values, I find the focal length to be [itex] +37.5 [/itex] cm
 

Answers and Replies

  • #2
41
2
Sorry, I figured it out... both are biconvex lenses therefore their focal lengths are positive. I believe I got confused with radius' of curvatures.. the solution given is the correct one.
 
  • #3
41
2
For a biconvex lens:

R1 > 0 and R2 < 0 (Radius of curvature)

not the focal lengths... :confused:
 

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