# Telescope

Telescope:
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:

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