Question

There are four convex lenses L₁, L₂, L₃ and L₄ of focal length 2, 4, 6 and 8 cm, respectively. Two of these lenses from a telescope of length 10 cm and magnifying power 4. The objective and eye lenses are respectively

Options

• L₁,L₂

• L₁,L₄

• L₂,L₃

• L₄,L₁

Answer 1

L₄,L₁

Explanation:

Let us assume, the focal length of eyepiece and objective lens of telescope be f_e and f₀, respectively.

The magnifying power of telescope,

M = `"f"_o/"f"_"e"` = 4

∴ f_o = 4f_e    ....(i)

The length of telescope is given by

L = f_o + f_e 

10 = f_o + f_e 

10 = 4f_e + f_e   ...[∵ f_o = 4f_e from Eq. (i)]

5f_e = 10

f_e = 2 cm

and f_o = 4 × 2 cm = 8 cm

Hence, L₄ is the objective lens and L₁ is the eye lens.