Answer
$2.1\ mm$
Work Step by Step
Given,
The angular magnification of an astronomical telescope $m_\theta = 36$
Diameter of the objective lens $d_{objective} = 75\ mm$
Minimum diameter for eye piece is given by:
$d_{eye piece} = \frac{d_{objective}}{m_\theta} $
$d_{eye piece} = \frac{75\ mm}{36} $
$d_{eye piece} = 2.1\ mm $
