Question

Explain why the hallway focuses to an image instead of point while constructing the Keplerian telescope?
Which lens is the most effective eyepiece? Why? assuming its objective vs. convergent

Answer 1

The focal length of objective lense( converging) will be greater  than the distance between the two lenses (Converging objective lense and diverging eyepiece lense). This is why the telescope is shorter in length.  Because the incident rays from the top and bottom of the object never cross at the focal point, the image is seen as being upright. This is a so called virtual image because the eye receives rays as if there was an image but in real there is no final image that can be seen on a screen.

There will be an image produced by the objective (converging lense) . This will be an inverted real image of the long distant object. This temporary image will be focussed just behind the focal point of eyepiece lense. (Suppose if this image is obtained on the focus, the final image will be getting at infinity). This image will be act as object for the eyepiece (diverging lense). The diverging lense produces the final image that we are able to see.

As convex lens create blurs and distortion, so its better installed concave lenses as eye pieces in order to focus more clearly. More over , the concave lense forms an upright image. But it will be diminished than object and also virtual.