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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 eyepiece 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:
Detailed Solution
In normal adjustment the final image is at infinity and is inverted. $m=-\frac{f_o}{f_e}=-\frac{20}{2\times10^{-2}}=-1000$. Tube length $L=f_o+f_e=20+0.02=20.02$ m. The aperture of the eyepiece is smaller than that of the objective. So (a), (b) and (d) are correct; (c) is wrong since the image is inverted.