Question 5. A lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since :
(1) a large aperture contributes to the quality and visibility of the images.
(2) a large area of the objective ensures better light gathering power.
(3) a large aperture provides a better resolution.
(4) all of the above.
Answer: Option (4)
Explanation:
An astronomical telescope is used to observe very distant and faint celestial objects,
so its objective lens must satisfy several important conditions.
A large aperture allows more light from distant objects to enter the telescope,
which increases the brightness and visibility of the image.
The light gathering power of a telescope is directly proportional to the area of the objective lens,
and a larger area means more photons are collected, making faint objects observable.
The resolving power of a telescope depends on the aperture of the objective lens.
A larger aperture reduces diffraction effects and enables the telescope to distinguish two closely spaced objects clearly.
A large focal length of the objective helps in forming a sharp image and
also provides higher magnification when used with an eyepiece of suitable focal length.
Since all the given statements correctly describe the advantages of a large focal length and
large aperture objective lens, the correct option is (4).