Question

In an astronomical telescope of refracting type, which statement is correct?
(A) Eyepiece has greater focal length.
(B) Objective has greater focal length.
(C) Objective and Eyepiece have equal focal length.
(D) Eyepiece has greater aperture than the objective.

Answer 1

The refracting type telescope is one of the types of optical telescope, that means it operates on convex lenses instead of mirrors for its impartial ability to form an image. Dioptric telescope is the other name for the refracting lens telescope. One of the main purposes of telescopes is to gather the light that is to focus the collected photons.

Complete step by step solution
The refracting telescope has two lenses, the eyepiece lens and the objective lens. Both the objective and eyepiece lens use convex lenses. Generally, the objective lens is larger than the eyepiece lens. The convex lens of the objective is used to obtain the real image of the object at a particular focal plane which is observed through an eyepiece. Parallel light source from a source of a distance passes through the focal point of the objective lens.

The eyepiece is placed in such a way that the lens matches with the focal point of the objective lens. The design of the refracting telescope was originally used in spy glasses and astronomical telescopes and now it is used in the camera’s long lenses. The size of the image that is produced by the objective lens is directly proportional to the focal length of the objective lens. The image that is formed will be larger with a larger focal length.

The brightness of the image produced by the telescope partly depends on the amount of light that a telescope can collect. The telescope's power of light gathering is directly proportional to the area of the objective lens. That is the telescope can gather more light with a larger objective lens.

Therefore, in an astronomical telescope of refracting type, the objective has greater focal length.

Hence, option (B) objective has greater focal length is the correct answer

Note: There are two types of telescopes. They are reflecting telescopes and refracting telescopes. The reflecting telescope uses curved mirrors. The inventor of the reflecting telescope uses two mirrors, that is primary mirror that is the objective mirror and the secondary mirror.