Hi guys ,
I am having difficulty with a Gaussian optics question and was wondering if anybody is able to offer me any kind of insight.
A real object 20cm from the first focal point of a coaxial system of two thin lenses forms a real image 45cm from the second focal point. If the focal length of the first lens is +12.5cm and the lens separation is 4cm, find the power of the second lens ?.
The answer given for this question is the second lens F2 is -6.86D.
My own answers are nowhere near this and I must be making some conceptual mistakes, my approach to this question is as follows:
the real object is placed -0.325m (-12.5cm + -20cm) from the front vertex of the front lens and has a vergence of -3.07D
the image formed right of the second lens is +0.575m (12.5cm+45cm) and has a vergence of +1.73D
the lens seperation d is 0.04m
then applying the conjugate foci equations
1/l = -3.07D=L1
L1+F1= L1' = -3.07 + 8.00D = +4.92D
L2 = L1'/1-dL1' = +6.125D
L2' = 1/0.575 = +1.73D
L2'-L2 = +1.73D - +6.125D = -4.39D = F2 hence the wrong answer
Any advice or insight would be greatly appreciated
Thanks in advance.
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