# Determining the focal length of a compound lens

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1. May 15, 2016

### Alex_Neof

1. The problem statement, all variables and given/known data
A biconvex lens, $L_1$, with focal length of magnitude $\left| f_1 \right| = 15$ cm is used with a second biconvex lens, $L_2$, with focal length of magnitude $\left| f_2 \right| = 25$ 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?

2. Relevant equations

$\frac{1}{f_c} = \frac{1}{f_1} + \frac{1}{f_2}$

3. 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

$f_1 > 0$

and

$f_2 < 0$?

Solution given is $+9.375$ 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 $+37.5$ cm

2. May 15, 2016

### Alex_Neof

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. May 15, 2016

### Alex_Neof

For a biconvex lens:

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

not the focal lengths...