question_answer

Two convex lenses A and B of an astronomical telescope having focal lengths 5 cm and 20 cm respectively, are arranged as shown in Fig. (i) Which one of the two lenses you will select to use as the objective lens and why? (ii) What should be the change in the distance between the lenses to have the telescope in its normal adjustment position? (iii) Calculate the magnifying power of the telescope in the normal adjustment position.

Answer:

                (i) The lens B should be used as objective because it has larger focal length and larger radius. (ii) In normal adjustment, the distance between objective and eyepiece \[={{f}_{0}}+{{f}_{e}}=20+5=25cm\] Distance required to be increased between the two lenses \[=25-15=\mathbf{10cm}\] (iii) Magnifying power of the telescope in normal adjustment, \[m=\frac{{{f}_{0}}}{{{f}_{e}}}=\frac{20cm}{5cm}=\mathbf{4}\]