telescope question?

Question

Why a two lens system is needed to make a telescope? (Make sure to refer to the object distance and the focal length of the objective length)

Answer 1

to bring focal lines back into parallel

Answer 2

I'm not sure "needed" is the right word, but this is the best description of why/how a simple refracting telescope suitable for astronomical observations works.

http://www.ifa.hawaii.edu/users/mickey/ASTR110L_S04/simplescope.html

The first answer, to make the the rays parallel again, isn't quite true, but does have an element of truth to it - many telescopes are designed so as to make that happen as it makes viewing and photographing easier.

Also, there are two quite independent functions that a telescope might have:

1. Increasing the amount of light from the object that enters the eye.

2. Providing an apparent magnification of the object.

A system of two lenses can do either one without doing the other, depending on the relative diameters and the relative focal lengths of the two lenses. Most astronomical telescopes are designed to do both, but to also increase resolution (which not independent but is related to both)

Here's a nice diagram of the details of an early telescope:
http://www.cavendishscience.org/phys/optics/telescop.htm

And, of course, wikipedia is always worth checking out:
http://en.wikipedia.org/wiki/Optical_telescope