# Two lenses problem, multiple interpretations, no true answer

1. Jun 24, 2011

### zimo

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

Two lenses, with 20cm focals are in 1m distance from each other.
A body is located 60cm from the right lens.

2. Relevant equations
I figured that the relevant equation should be:
$\frac{n1}{u} + \frac{n2}{v} = \frac{n2 - n1}{r}$
but perhaps a I'm wrong.

3. The attempt at a solution

Using the equation above, when u = 60, n1=1(air), n2=1.5(glass) and r=f*2=40
and then again for u=v' (that we got at the first use of the equation) and my answers vary on my interpretation of the problem - my final answers was 20cm right to the right lense and v=51.42cm to the left of the left lense...
They both were wrong.

2. Jun 24, 2011

### zimo

I understood my mistakes:
a. the correct equation to use is the simpler lens equation (although we can use the above one, but no need)
b. on the second iteration we need to take the new u from the old v but to do the conversion 100-u=u(new) because the new reference image is between the two lenses.

now the final solution is 28cm (and correct, according to the final solution sheet that our lecturer gave us)