Question

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 ______.

Options

• 20 cm

• 30 cm

• 10 cm

• 15 cm

Answer 1

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 20 cm.

Explanation:

∵ f₀ + f_e = 30

And magnification, m = `"f"_0/"f"_"e"`

2 = `"f"_0/"f"_"e"` ⇒ f₀ = 2f_e ⇒ f₀ +  `"f"_0/2` = 30 

∵ f₀ = 20 cm