Question
Fri May 30, 2008 By:

CAN A LENS BE CONVERGING IN ONE MEDIUM AND DIVERGING IN OTHER MEDIUM?

Expert Reply
Tue June 03, 2008

yes.it can.

first of all,know that a converging lens always has a +ve focal lenght and a diverging lens has a -ve focal length.

lens maker's formula tells us that:

1/f=(n2-n1)/n1 * [1/R1-1/R2]
where n1 is refractive index of surrounding medium and n2 is RI of the lens material.
R1&R2 are the radii of curvatures of first aand seconf refracting surfaces.
Now,suppose you take a convex lens and place it in air.
n2>n1
therefore (n2-n1)/n1 ___________+ve

[1/R1-1/R2]___________is always +ve

so,the focal lenght[f] is also +ve.the lens is converging type.

but if the same lens is placed in medium of higher RI,
n2<n1
which implies (n2-n1)/n1 _________-ve

so,focal lenght is-ve and the lens is diverging type.............

regards

toppers expert

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