# Doubt with power of lens with 2 media

1. Aug 16, 2010

### sachin123

This is more of a general question.

Suppose there is a biconvex lens(Radii are R1 and R2) with Refractive index =n2,it is bounded on 2 sides by media of RI=n1,n3.
Now if I have to find its focal length,
1/f=(n2-n1)/R1+(n2-n3)/R1.
Is it correct?

Suppose it were correct,
If there is a biconvex lens with radii of curvatures R,with refractive index(RI)=n2 and surrounded by medium of RI=n1,then its power is,(n2/n1-1)(2/R)(Lens maker formula).Correct?
But if we consider individual surfaces and use Power=(n2-n1)/R for 1rst and (n1-n2)/R for the second surface,and add them,I get Power=(n2-n1)(2/R).The two are different.
Now where did i go wrong?

Wrong?
Or should it be used like(I read in many places):
1/f=((n2/n1)-1)/R1+((n2/n3)-1)/R1.
or
1/f=((n2/n1)-1)/R1+((n3/n2)-1)/R1.
Please explain.
I am seriously confused:(.
I would explain more if didn't understand.
Thanks

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