A reflecting type telescope has a concave reflector radius of curvature 120 cm. Calculate the focal length of eyepiece to secure a magnification of 20 .

Explanation

Data supplied ,
m = - 20 , R = 120 cm, `f_(0) = (R)/(2) = (-120)/(2) = - 60 ` cm , `" m" = (f_(0))/(f_(e))`
`f_(e) = (f_(0))/(m) = (-60)/(-20) = 3 ` cm