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.
The pupil is the hole located at the center of the iris (the colored part of your eye) that allows light to enter the retina (tissue lining the inner surface of the retina). Muscles attached to the iris either contract or stretch to widen or shrink the diameter of the pupil.
When you are in a dimly lit room the iris stretches to dialte the pupil as much as possible to allow more light to enter the eye to permit you to see better in dimly lit conditions. In bright conditions the iris contracts to shrink the pupil because there is more light available than is needed to allow you to see.
The easiest way to make a person's pupil dialate is to shine a light in their eye, let the pupil contract, than move the light away and watch the pupil dialate. Doctor's will do this during a regular checkup to check the health of your eyes.
Some drugs and illnesses will also make the pupils dialate.
tell your teacher to stay for a while inside a brightly lighted room.. due to excess of light, the pupil's size wil decrease..
then tell your teacher to go inside a dimly lighted room.. then the pupils will dilate..