How to determine a lens width?

[clalburn1420]

I am wondering... say an object was put directly on a lens and you wanted the projected image to focus at around 3 cm on the other side. How do you determine how wide the lens has to be?
(please see attachment)

 

[earlofwessex]

"how wide" in itself is not enough to specify this, since the angle of incedence depends on the curvature of the lens. you must look for a lens wth the correct focal length, where
1/f = 1/u + 1/v (if I'm remembering correctly)

the top bit of this page explains...
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html

 

[clalburn1420]

Ah, so it is also determined by the shape. However, i expect it would need to be a convergent lens type and assume it needs to be like a () shaped lens. The problem is however, that this equation can help find focal length but not describe how the lens itself will be shaped.

 

[Andy Resnick]

Placing an object directly in contact with a lens (any lens) is not a good way to form an image any finite distance away from the lens. Why do you want to do this?

 

[clalburn1420]

Simply curious if its possible. I'm not sure it is, which is why I'm wondering. What about if the object is a very small distance from the lens, like say 1 mm. Can you focus the image on the other side 3 cm away?

 

[Andy Resnick]

Sure, if the focal length of the lens is correct. That's how a microscope objective works.