Introduction
Have you ever considered taking a nighttime nature walk? Would you wait until there is a full moon so you could benefit from sunlight reflected from the moon—or would you rather take a flashlight? Do you think trees would look black, green or gray in the dark? Try this activity to examine your night vision and prepare for your next nighttime adventure!
Background
Sight begins when light enters the eye. This light triggers light-sensitive cells in the retina at the back of the eye. As a result signals zoom along the optic nerve to the brain. The brain then makes sense of the signals, giving us the experience of seeing.
The pupil is the opening in the middle of the front of the eye that allows light to enter. Humans have round pupils. They appear black because light almost never escapes through them. The colored part around the pupil called the iris adjusts the size of the pupil. Its main function is to regulate the amount of light that enters the eye. In dim light the pupils dilate (open wider) so more light can enter. Switch to bright light and the pupils automatically contract. This is the result of a nerve signal generated in the back of the eye triggering the muscles in the iris. Because some nerve connections cross over to the other eye, both pupils contract in unison.
