Seeing in the Dark

Introduction

Have you ever considered taking a nighttime nature walk? Would you wait until there is a full moon, so you could profit from sunlight reflected on the moon, or would you rather take a flashlight? Do you think trees would look black, green, or grey in the dark? Do this activity to examine your night vision and prepare for your next nighttime adventure!

Materials

• Flashlight that shines white light.
• Flashlight that shines red light. You can also hold a translucent red object like a translucent red food container lid in front of the white flash to make it shine red light.
• Dark room. Almost pitch-black works best. You can also do it outdoors at night, away from street lamps or other sources of light.
• Bag with markers, pencils, and pens of different colors.
• A helper whose eyes you feel comfortable looking into, or a mirror.

  Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies

What Happened?

Did the pupils expand when exposed to an environment with little light and contract when there is plenty of light? Were you able to recognize objects in dim light, but unable to recognize their color? That is what our eyes are designed for.

The human eye is designed such that it can detect lines, contrast, and movement in dim light, but it needs plenty of light to see color. That is why in dim light conditions, you recognized objects, but failed to detect their color.

Pupil size changes to optimize vision in a large range of light conditions. When there is plenty of light, a smaller opening in the eye—or a smaller pupil—protects the back of the eye from getting damaged. In dim light, the pupils dilate to allow as much light in as possible. That is why you see the pupils change size when light conditions change. Red light does not trigger pupil contraction as much as other colors of light, making a red flashlight ideal for enjoying the night landscape.

Digging Deeper

Sight begins when light enters the eye. This light triggers light-sensitive cells in the back of our eyes and as a result, signals zoom along the optic nerve to the brain. The brain makes sense of the signals, giving us the experience of seeing. The human eye has two types of light-sensitive cells: cones and rods. The rods are extremely efficient: a tiny amount of light can trigger them. They are responsible for our night vision. They detect lines, contrast, and movement, but cannot distinguish color. The cones are responsible for color vision, but these need plenty of light to get activated. That is why in dim light conditions, you recognized objects, but failed to detect their color.

The pupil is the opening in the middle of the eye that allows light to enter the eye. 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 contraction is the result of a small fraction of the nerve signal generated in the back of the eye triggering the muscles in the iris. As some nerve connections cross over to the other eye, both pupils contract in unison.

Pupil size does not only change with the intensity of light falling into the eye. It is also influenced by your emotional state. When you become overwhelmed with feelings like fear, anger, pain, love, or admiration, your pupil size changes. Some studies show that people even react to the pupil size of others in their environment. A head injury, illness, or drugs can also influence pupil size.

For Further Exploration

• Find an adult to accompany you on a night outing to an area where there is very little light pollution. How easily can you do a night walk with your flashlight off, with your flashlight on the red light setting, and with the flashlight on the white light setting? How is the experience different? Does the number of stars you can observe depend on the light you use?
• By wearing an eye cover over one eye while exposing the other eye to bright light, you can investigate how bright light in one eye changes the pupil size of the other eye. Do both pupils work in unison when they contract or expand, or does it happen independently?
• Not only light influences the size of your pupils; the way you feel also affects the size of your pupils. Can you design a test to see if pupils become larger or smaller when someone sees something they like a lot? What do your findings tell you? Look online and see if your findings coincide with findings from pupillometry.

Related Resources

Project Ideas

Activities

Lesson Plans

Links

• Blog Post: Binocular Vision Eye Science
• Blog Post: The Eyes Have It!
• Blog Post: A Trick of the Eye for Halloween
• Scientific American: Pupils Dilate or Expand in Response to Mere Thoughts of Light or Dark

Careers