CBSE Class 12 · 2017 · Delhi · Geometrical OpticsShort Answer

Why should the objective of a telescope have a large focal length and a large aperture? Justify your answer.

Solution

∙

\textbf{Large focal length of objective:} the magnifying power of a telescope is

m = \frac{f _{o}}{f _{e}}

, so a large

f_{o}

gives a large magnification.

∙

\textbf{Large aperture of objective:} resolving power

= \frac{D}{1.22 λ}

, so a large aperture

D

gathers more light (brighter image) and resolves finer detail.

CBSE Class 12 Board · 2017 · Delhi · Subjective