# Answer to Question #25649 in Optics for dellu

Question #25649
if focal length of convex lens is f and it is cut into two equal part what is focal length of each part?
1
2013-03-07T08:55:01-0500
There can be two ways to cut a convex lens into two equal parts - one, along the principal axis and another, perpendicular to the principal axis. If the lens is cut along the principal axis, then there will be NO change in the focal length of the lens. But, if it is cut perpendicular to the principal axis, then the focal length will be twice the original one.

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Assignment Expert
17.07.14, 17:38

Dear Fatima,

The focal distance f of a lens can be calculated using a lensmaker's equation:

1/f = (n - 1)(1/R_1-1/R_2),

where n is a refractive index, R_1 and R_2 are curvature radii. For a biconvex lens the R_2 is taken negative. If |R_1| = |R_2| = R, we have

f = R/(2*(n-1)).

If one cuts lens perpendicularly to the principal axis, the obtained lens will be convex only on the one side and flat on another side: R_1 = R and R_2 = infinity. So,

1/f_1= (n-1)/R   =>f_1=R/(n-1)=2f.

fatima
19.06.14, 10:28

explain plz??
infact i want to know is there any relation between diameter and focal length?