An astronomical refracting telescope is being used by an observer to observe planets in normal adjustment. The focal lengths of the objective and eye piece used in the construction of the telescope are 20 m and 2 cm respectively. Consider the following statements about the telescope: (a) The distan…

Question

An astronomical refracting telescope is being used by an observer to observe planets in normal adjustment. The focal lengths of the objective and eye piece used in the construction of the telescope are 20 m and 2 cm respectively. Consider the following statements about the telescope:
(a) The distance between the objective and eye piece is 20.02 m
(b) The magnification of the telescope is (-) 1000
(c) The image of the planet is erect and diminished
(d) The aperture of eye piece is smaller than that of objective
The correct statements are:

Accepted Answer

(1) (a), (b) and (d). Given: fₒ=20 m=2000 cm, fₑ=2 cm, normal adjustment. Idea: in normal adjustment the final image is at infinity, so the tube length is fₒ+fₑ and the angular magnification is -(fₒ)/(fₑ). Tube length =20 m+2 cm=20.02 m, so (a) is true. m=-(2000)/2=-1000, so (b) is true. The minus sign says inverted and |m|=1000 says magnified in angle, so (c) is false. The objective is made wide to gather light while the eye piece matches the pupil, so (d) is true. The correct statements are (a), (b) and (d).

Step-by-step solution

Why the other options are wrong