Determine Which Light Color Has the Least Effect on Night Vision

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Author: Janice VanCleave

What You Need to Know

Optical instruments are devices, such as telescopes, designed to aid human vision. The light-gathering power of optical instruments is the amount of light the instrument can collect. Light amplification is the process by which objects viewed through an optical instrument appear brighter than when viewed with the unaided eye. The aperture of an optical instrument is the opening through which light enters. The aperture size indicates the light-gathering power of the instrument. The pupils of your eyes have a variable (changing) aperture. They contract (get smaller) in the light and dilate (get larger) in the dark. In the light, your eye is said to be light-adapted and in the dark it is dark-adapted, also called night vision.

How Does Dark-Adaptation Work?

The aperture of your eye is the pupil. White light causes your pupils to contract or get smaller, restricting the amount of light that enters the eye. This is called having light-adapted eyes. In the dark your pupils dilate or get larger, which allows more light to enter. This is called having dark-adapted eyes. Your pupils are like the aperture of a telescope: the larger the aperture, the more light-gathering power. This is important when studying the night sky. With a larger aperture or pupil, your eyes can gather more starlight so you can see more stars.

How Does Dark-Adaptation Work?

What Does This Have to Do with Colored Light?

While studying the night sky, astronomers may need to read star maps and/or take notes without losing their night vision. Would colored light have less effect on night vision than white light? Would the shades of the different colors have any effect?

Real-Life Science Challenge

There are two kinds of light-detecting cells on the back of your eye: cones and rods. The cones occupy a small spot centrally located on the back of your eye, and the rods surround the cones and cover a much greater area. When you look straight ahead, light enters your eyes and falls on the cones, which are cells for high-level light that specialize in color perception. When you look to the side (up, down, left, or right), light enters your eyes and falls mostly on the rods, which are sensitive to low-level light. Astronomers have discovered that when viewing objects with low-level light the best view is achieved by looking off to the side. This is because the light falls on the rods instead of the cones. This method of viewing is called averted vision.

Fun Fact

Having dark-adapted eyes is called having night vision. When you move from a lighted area to a dark area, at first you can hardly see. After a few minutes, changes occur in your eyes and you can see better. In about 30 minutes to an hour, the changes are complete and your vision is even better. Although your vision is not as good as in the light, it is the best it will be in the dark. You now have night vision.

One flash of white light, such as from a flashlight, instantly changes dark-adapted eyes to light-adapted eyes. But turning off the light doesn't instantly change the eyes back to their dark-adapted state. It takes another 30 minutes to an hour to get back your night vision.


Now, start experimenting with colored lights and night vision.


  • A flashlight can be used as a light source.
  • Colored transparent cellophane or plastic report folders can be used over the white light source to produce colored light.
  • Design a way to measure the degree of change each type of light makes to dark-adapted eyes.
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