Arrange the following lenses in increasing order of their typical foc…

The correct answer is Option (3) → (A), (C), (D), (B)

The design of each instrument decides its focal lengths. A compound microscope needs a very short objective focal length for a large magnification of a nearby object. An astronomical telescope needs a very long objective focal length, since its magnifying power is \( f_{o}/f_{e} \). The eyepieces of both are simple magnifiers of a few centimetres.

    \( \text{(A) Objective lens of a compound microscope: a few mm (about 0.5 to 2 cm)} \)
    \( \text{(C) Eyepiece of a compound microscope: about 2.5 to 5 cm} \)
    \( \text{(D) Eyepiece of an astronomical telescope: a few cm (for example about 6 cm)} \)
    \( \text{(B) Objective lens of an astronomical telescope: about 1 m or more} \)

Increasing order of focal length:

    \( \text{(A)} \lt \text{(C)} \lt \text{(D)} \lt \text{(B)} \)

NCERT (2026-27):
The Microscope – “… Clearly, to achieve a large magnification of a small object (hence the name microscope), the objective and eyepiece should have small focal lengths. In practice, it is difficult to make the focal length much smaller than 1 cm. Also large lenses are required to make L large. For example, with an objective with \( f_o = 1.0 \) cm, and an eyepiece with focal length \( f_e = 2.0 \) cm, and a tube length of 20 cm, the magnification is

    \( \displaystyle m = m_o m_e = \frac{L}{f_o}\,\frac{D}{f_e} = \frac{20}{1}\times\frac{25}{2} = 250 \)”

Telescope – “… It also has an objective and an eyepiece. But here, the objective has a large focal length and a much larger aperture than the eyepiece.”

Exercises – “… 9.13 A small telescope has an objective lens of focal length 144cm and an eyepiece of focal length 6.0cm. What is the magnifying power of the telescope?”