Question

In an astronomical telescope the focal lengths of objective and eyepiece should respectively be

• A. large and small
• B. small and large
• C. equal
• D. too small and too large

Answer 1

The correct option is A large and small

In an astronomical telescope, two converging lenses (or) convex lenses of different focal lengths are used.

$\text{Objective lens:}$
The converging lens having larger focal length with large aperture, so that it can gather sufficient amount of light from the distance objects i.e., stars and planets.

$\text{Eyepiece :}$
The converging lens having small focal length with smaller aperture, so that whole light of the telescope may enter the eye for distinct vision.

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(A\right)$ is the correct answer.

Why this question? To give the basic idea of construction of an Astronomical telescope.