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Question:

A convergent doublet of separated lenses, corrected for spherical aberration, has a resultant focal length of 10cm. The separation between the two lenses is 2cm. What are the focal lengths of the component lenses?

10cm, 12cm

12cm, 14cm

16cm, 18cm

18cm, 20cm

Solution:

By using formula f = (f1f2)/(f1 + f2 - d)
By trial and error method
Putting all option values one by one
For A we get f = 10 cm which is given
So option A is correct