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Cool goIITian

Joined:	5 Aug 2007
Post:	38

3 Jan 2009 23:02:13 IST

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350

Utter confusion?Focal length of biconvex lens by ray optics and Lens makers formula

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For a thin convex lens when an object is placed at center of curvature of a biconvex lens (both radii same) , the image is formed at center of curvature on other side. Similarly all rays parrallel to principal axis pass through focus after refraction. All this has been tought through ray diagrams. The result has been proved grometrically also that for paraxial rays focal length f=R/2.

Now if I use the lens makers formula for same biconvex lens, I get f=R, Why not f=R/2 ? Where is all the mistake I am making.

Immediate help requested .

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Comments (2)

sajal kumar

Cool goIITian

Joined: 26 Dec 2008

Posts: 62

3 Jan 2009 23:33:05 IST

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look it is nowhere written that f = R/2 for a lens...from lens maker formula u cannot get f = R/2 as focal lenght also depends on the refractive index of lens...... and from geometry we get f = R/2 for a spherical mirror....not for lens

Sagar Saxena

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Joined: 8 Oct 2008

Posts: 7221

5 Jan 2009 00:28:57 IST

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yes dear..

Focal length not only depend upon the radius of curvature of a lens,it also depend upon the refractive index of a substance or material.........

for mirror ,v uses f = r/2 but not for lenses while calculating the focal length of a lens

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