Question

Draw a ray diagram of a refracting astronomical telescope when final image is formed at infinity. Also write the expression for its angular magnification (magnifying power).

Draw a labelled ray diagram of an astronomical telescope when the final image is formed at infinity. Write the expression for its magnifying power.

Answer 1

In an astronomical telescope, the objective lens forms an intermediate real image, and the eyepiece lens magnifies it.

• The object (e.g., a distant star) is placed far away, so the incoming rays are parallel.
• These parallel rays first pass through the objective lens, which converges them to form a real, inverted image at its focal point (F_o).
• The eyepiece lens is then positioned such that this intermediate image lies at its focal point. As the eyepiece lens diverges the rays, they appear to originate from infinity, creating the final image that is seen by the observer.

Magnifying power (Angular Magnification):

1. When the image is formed at a finite distance (e.g., at distance D):
The magnifying power is given by the formula:

M = `- f_o/f_e  (1 + f_e/D)`

This formula takes into account the image formed at a distance D (closer than infinity).

2. When the image is formed at infinity (which is typical for astronomical telescopes):
The magnifying power simplifies to:

M = `- f_o/f_e`

This is the standard expression for magnifying power when the final image is at infinity.