Keplerian Telescope
The Keplerian Telescope, invented by Johannes Kepler in
is an improvement on Galileo's design. It uses a convex lens as the eyepiece instead of Galileo's concave one. The above is a schematic design of the Keplerian Telescope, where the center of the objective and eyepiece lenses are
and
, respectively. When the distance between them is
, the image of the moon becomes the sharpest. If the focal length of the eyepiece lens is
then which of the following statements is NOT correct?
a) The focal length of the objective lens must be larger than
b) The image shown by the eyepiece lens is a virtual image.
c) The distance between the image created by the objective lens and the center of the eyepiece lens is smaller than
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The aim of the Telescope is to achieve a magnified(angular) image for an object that is at a very large distance from the telescope. And for a relaxed eye the image should be at least at least distance of distinct vision. In this case we prefer to make the final image as far as possible. Therefore the faraway object has to be focused near the focus of eyepiece(so that we obtain magnified and image far away). And for that we use an objective which does this by focusing the object at the focus of of eyepiece , Hence the focal length of objective should be less than L. (Please correct me if I am wrong somewhere)