Telescope Lenses: Focal Lengths 20cm/1cm/100cm/5cm/10cm/2cm/25cm/75cm

[Bostonpancake0]

Which two of the following lenses could be used to construct a telescope?

Focal lengths: 20cm/1cm/100cm/5cm/10cm/2cm/25cm/75cm


(a.) That magnifies 20x and has a length of approx 1.0m
(b.) magnifies by 15x
(c.) magnifies 50 x

I used magnification=telescope focal length/eye piece length (Lense)

but am confused when they save two of the following lenses can be be used??

 

[Simon Bridge]

but am confused when they save two of the following lenses can be be used??

How so: you know that telescopes use more than one lens right?
The formula kinda suggests that you do.

Where does the length of the telescope come into this?

 

[Avaron Cooper]

but am confused when they save two of the following lenses can be be used??

the length of the telescope at normal adjustment when two lenses are used is kind of a special case don't u think?

 

[andrevdh]

http://science.howstuffworks.com/telescope.htm