Determine the minimum multiplication of a telescope with diameter of objetive `D = 5.0cm` with which th resolving power of the objective is totally employed if the diameter of the eye's pupi is `d_(0) = 4.0mm`.

Explanation

By definition, the magnification
`T = ("angle subtended by the image at the eye")/("angle subtended by the object at the eye") = (psi')/(psi)`
At the limit of resolution `psi = (1.22lambda)/(D)`
where `D =` diameter of the objective
On the other hand to be visible to the eye `psi' ge(1.22lambda)/(d_(0))`
where `d_(0)=` diameter of the pupil
Thus to avail of the resolution offered by the telescope we must have
`T ge (1.22lambda)/(d_(0))//(1.22lambda)/(D) = (D)/(d_(0))`
Hence `T_(min) = (D)/(d_(0)) = (50mm)/(4mm) = 12.5`