A telescope consists of two convergent lenses (objective and eyepiece). In a telescope the objective has a longer focal length and a larger aperture.


Light from a distant object enters the objective and a real image is formed at the focal point of the objective. The eyepiece then further magnifies the image. In case of a telescope, magnification is defined in terms of angles subtended by the image and the object.

$ \displaystyle M = \frac{Angle \; subtended \; at \; eye \; by \; image }{Angle \; subtended \; at \; eye \; by \; object} $

$ \displaystyle M = \frac{\beta}{\alpha} $

(a)If the final image is formed at infinity i.e. for normal adjustment of the telescope,

$ \displaystyle M = -\frac{f_o}{f_e} $

(b) When the final image is formed at the least distance of distinct vision:

$ \displaystyle M =-\frac{f_o}{f_e} (1 + \frac{f_e}{D}) $

Also Read :

Reflection of Light at a Plane Surface
Reflection at Spherical Surface & Mirror Formula
Lateral Magnification
Refraction of Light , Laws of Refraction , Relation between real and apparent depth
Refraction through Number of media
Total internal reflection
Refraction through a prism
Angle of minimum deviation & Prism Formula
Refraction at Curved surface
Lens maker’s formula
Combinations of Lenses
Simple Magnifier
Compound Microscope