CBSE Class 10 Answered
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.
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,
which implies (n2-n1)/n1 _________-ve
so,focal lenght is-ve and the lens is diverging type.............
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