Question

Draw aray diagram to show the image formation of a distant object by a refracting telescope. Write the expression for its angular magnification in terms of the focal lengths of the lensesused. State the important considerations required to achieve large resolution and their consequent limitations.

Answer 1

Angular magnification when the final image is at infinity (Normal Adjutment):
`M=(f_(0))/(-f_(e))`
Angular magnification when the final image is formed at least distance of distinct vision :
`M=(-f_(0))/(f_(e))(1+(f_(e))/(D))`
Resolving power of telescope is given by :
`R.P.=(D)/(1.22 lamda)`
Resolving power of telescope can be increased by increasing the diameter (D) of objective lens. Refracting type telescope has many limitations like chromatic and spherical abserration.