3.4 Galileo telescope (opera glasses)
A convex lens of long focal length ƒ1 (+300 mm, for example)
and a concave lens of short focal length ƒ2 (e. g. -50 mm) are
set up at a distance of ƒ1 – |ƒ2-| (Fig. 6).
Through the concave lens we can see distant object magnified
and the right way up.