310 Aptitude Test Problems in Physics
that with all possible reflections, the given ray
will remain in the constructed plane (Fig. 245).
After the first reflection at the conical surface,
the ray AB will propagate as if it emerged from
point A', viz, the virtual image of point A. The
necessary condition so that none of the rays emerg-
ing from A ever gets on the mirror is that point A'
must not be higher than the straight line OC, viz.
the second generator of the cone, lying in the plane
of the ray (point 0 is the vertex of the conical sur-
face). This will be observed ifLA'0D+ LA0D-1- LAOC =3 -Tcc > 180°. -Consequently,
Amin > 120°•