How can I see my teacher’s pupils dilate without any instruments?

Very simply, the pupil, which is an opening in the center
of the eye, allows light to enter the eye. It is a round opening in humans, controlled
by the iris, which contains muscles that cause the pupils to widen when light is scarce
or become smaller, when light is abundant. This is a reflex action controlled by the
brainstem. If you shine a bright light at your teacher, her pupils will become smaller.
If she closes her eyes for a few minutes, and opens them quickly, the pupils contract
also. Likewise, if she is in a brightly lit room and then the lights go out, her pupils
will become larger. This reflex action allows the amount of light to be regulated as it
enters the eye, depending on how much light is available in the surroundings of the
individual.