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

Asked by  | 30th May, 2008, 11:55: AM

Expert Answer:

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

Aswered by  | 3rd Jun, 2008, 11:04: PM

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