# Convex lens and focal length

## Main Question or Discussion Point

1)Why when a convex lens having radius of curvature R, focal length f and refractive index μ is

bisected horizontally along the principal axis its focal length remains the same, whereas when

it is bisected vertically focal length becomes 2f? Does the same thing happens in the case of

concave lens too?

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tiny-tim
Homework Helper
Hi Alche!

Tell us what you think, and then we'll comment.

Tell us what you think, and then we'll comment.
Well, I think when we bisect the convex lens vertically its radius of curvature becomes R/2 so the focal length becomes 2f.

And I am sure the same things happens with concave lens too :tongue:

tiny-tim
Homework Helper
Hi Alche!

To be honest, I can't remember the formula ,

but yes the vertical cut changes the focal length because it changes the shape of the surface the light refracts through (and so yes, it's the same for a convex lens) …

the horizontal cut doesn't change the shape of the surfaces, it only makes the lens smaller, so the focal length stays the same.

Thanks Sir

focal length of the lens f = R/2, where R is the radius of curvature of the given lens