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 observes distant celestial bodies. It comprises two convex lenses:

• Objective lens (characterized by a large focal length)
• Eyepiece lens (characterized by a shorter focal length)

When the final image is positioned at infinity, the intermediate image, formed by the objective lens, is located at the eyepiece's focal point. Consequently, the final image produced is a virtual, magnified, and inverted image at infinity.

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