Calculating Telescope Length: Refracting Telescopes

[edoarad]

in a refracting telescope, the lens's diameter is 1.02 m and its focal length is 19.4 m.
how long must the telescope be to get an image, 40 times bigger?

i would also like to get a link to a site explaining this kind of elementary optics.

 

[edoarad]

just saw the warning, please don't delete my posts :S they are in a close enough format i think..