Question

Explain with the help of a labelled ray diagram the formation of final image by an astronomical telescope at infinity. Write the expression for its magnifying power.

Hint:

To get the final image at infinity in a telescope, place the intermediate image formed by the objective at the focus of the eyepiece. This gives relaxed viewing with maximum comfort.

Answer 1

Working Principle:
An astronomical telescope is used to view distant celestial objects. It consists of two convex lenses:

• Objective lens (large focal length)
• Eyepiece lens (shorter focal length)

When the final image is formed at infinity, the intermediate image formed by the objective lies at the focus of the eyepiece. The final image is then a virtual, magnified, and inverted image at infinity.

Magnifying Power (M):
For final image at infinity, \[ M = \frac{f_o}{f_e} \] where $f_o$ = focal length of objective, and $f_e$ = focal length of eyepiece.