In normal adjustment, for a refracting telescope, the distance between objective and eye piece is 30 cm . The focal length of the objective, when the angular magnification of the telescope is 2 , will be:

Step-by-step solution

Let f 0 be the focal length of objective and f e is the focal length of eyepiece. Given here, f 0 + f e = 30 . Angular magnification of telescope is given by m = f 0 f e . So, 2 = f 0 f e ⇒ f 0 = 2 f e Then, we have f 0 + f 0 2 = 30 ⇒ 3 f 0 2 = 30 Or, f 0 = 20   cm .