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Reflectors: Do Cats’ Eyes Glow in the Dark?

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Author: Janice VanCleave

Problem

Do cats' eyes glow in the dark?

Materials

  • Scissors
  • Construction paper
  • Empty coffee can or oatmeal canister (inside bottom must be shiny)
  • Masking tape
  • Flashlight

Procedure

  1. Cut a circle from the construction paper large enough to cover the end of the can.
  2. In the center of the paper circle, cut a long oval opening.
  3. Tape the paper circle over the open end of the can. This is a model of a cat's eye.
  4. In a darkened room, hold the can at arm's length and at eye level in front of you with the paper end facing you.
  5. Look toward the oval opening in the paper, and record your observations.
  6. Hold the flashlight in front of your face with the light pointing at the oval opening in the paper circle.
  7. Again look towards the oval opening, and record your observations.

Reflectors

Reflectors

Results

The can is not very visible in the darkened room. When a flashlight is shined at the can, the shiny bottom of the can as well as the paper glows.

Why?

Cats' eyes do not "glow" in the dark. The glow from the animal's eyes is due to the reflection of external light. The back of each cat's eye has mirror like cells that, like the bottom of the coffee can, reflect light. These cells are filled with a chemical called guanine that reflects even the smallest amounts of light and thus floods the eyeball with light, causing them to glow. The eyes do not appear to glow during the day because the oval-shaped slit in the animal's eyes (called the pupil) is only slightly open. Any light reflected during the day is not noticed because of the brightness of the sun's light. At night, the pupil opens exceptionally wide, allowing more light from a flashlight or other external light source to enter and be reflected from the back surface of the eye. The reflected light makes the eyes seem to glow in the dark.

Let's Explore

Does the size of the pupil (dark opening in the eye) affect the glow of the cat's eye? Change the size of the oval slit in the paper circle and repeat the experiment. Science Fair Hint: Display the paper circles used in order of their ability to produce a glow.

Reflectors

Show Time!

  1. Why does the pupil of the cat's eye open more at night? In dim light or darkness, the muscles in the front of the eye of all animals relax, causing the opening in the eye to enlarge. To observe the effect that light has on the size of an eye's pupil, sit in a brightly lighted room for two minutes. Keep one eye tightly closed and the other eye open. Observe the pupil of the open eye by looking in a mirror. Open the closed eye and immediately observe the size of the pupil. Notice and record the difference in the pupil sizes of each eye. Use photographs of you as you perform this experiment. Drawings showing the size of your pupils can also be part of your project display.
  2. Humans and other animals with good day vision have a thin layer of black tissue, called choroid, on the back surface of their eyes. Does the choroid layer reflect light? Cover the shiny bottom of the can with a piece of black construction paper and repeat the experiment. Use the materials and the results as part of a project display.

Reflectors

Check it Out!

Nocturnal hunters (animals that hunt at night) tend to have a long, oval-shaped pupil, while daytime prowlers have a round pupil. Find out how the shape of the pupil affects an animal's ability to see.

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