Question

You are given the following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct a compound microscope?

Lenses	Power (D)	Aperture (cm)
L1	3	8
L2	6	1
L3	10	1

Answer 1

For a compound microscope, an objective is so chosen that its aperture is small and focal length is very small whereas an eyepiece is so chosen that its aperture is large and focal length is moderate. Now,
focal length of L₁, f₁ = 1/3 = 0.33 m
focal length of L₂​, f₂ = 1/6 = 0.166 m
focal length of L₃, f₃ = 1/10 = 0.1 m

Therefore, from the above calculation of focal length and given aperture, it shows that L₁ will be suitable for eyepiece and L₃ for objective.