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Sliding Gray Step

Science Snack
Sliding Gray Step
Don't believe everything you see.
Sliding Gray Step
Don't believe everything you see.

With this simple Snack, you can make one shade of gray look like two by putting it against different-colored backgrounds. The difference will amaze you!

Tools and Materials
  • One sheet 8-1/2 x 11 white paper (or use white posterboard if you want to make the Snack a little more durable)
  • One sheet black construction paper (size not critical, since you will be cutting it into strips)
  • Ruler
  • Scissors or paper cutter
  • Glue stick
  • Gray duct tape from a roll approximately 2 inches (5 cm) wide; alternatively, one sheet of gray construction paper the color of gray duct tape. We use duct tape since it’s easier to obtain, but paper is easier to cut.
  • One sheet 8-1/2 x 11 transparency film
  • Paper clips
  1. Cut eight strips from the black construction paper, each 1/2 inch (1.3 cm) wide and 8 inches (20 cm) long.
  2. Use the glue stick to glue the black strips to the white paper, forming horizontal bars 1/2 inch (1.3 cm) apart from each other (see photo below).
  3. Cut 8 strips of duct tape approximately 2 inches (5 cm) long (the width of the roll) and 5/8 inches (1.6 cm) wide.
  4. Place the overhead transparency film over the black-and-white striped pattern, and use paper clips to hold it in place.
  5. Carefully place four of the gray pieces of duct tape on the transparency sheet so they’re on top of the middle four black bars. Their ends should be approximately 1 inch (2.5 cm) from the left edge of the sheet. Then, about an inch to the right, place the other four tape strips on white bars, as shown in the photo below.
  6. Remove the paper clips so you can move the transparency film freely over the black-and-white bars.
To Do and Notice

Notice that, even though they’re the same color, the gray strips between the white bars appear lighter, while the gray strips between the black bars appear darker.

Slide the transparency slowly up and down, and notice how the shades of gray appear to change as the strips move to positions between black bars instead of white bars, and vice-versa. Notice that the strips that were light become dark, and vice-versa.

Slide the transparency so the gray strips are half on a white bar and half on a black bar. Notice that the two sets of gray strips now appear the same.

What’s Going On?

Both sets of strips are the same shade of gray: They came from the same roll of tape. When you change the position of the strips relative to the black and white bars, however, they seem to get lighter or darker. This illusion is not fully understood, but it seems to have something to do with the way your eye determines relative shading.

Your eye focuses an image of the striped pattern on your retina, the layer of light-sensitive cells at the back of your eye. Nerve cells in the retina begin processing the light and dark information in two different ways.

Some nerve cells in your eye look at the big picture, receiving information from a large area of the retina. These cells blend light from several bars and react as if the light were mixed together. When the gray strips are between the white bars, you see pale gray—a mixture of white and gray. When the gray strips are between the black bars, you see dark gray—a mixture of black and gray.

At the same time, other nerve cells in your eye receive information from a small area of the retina, and are sensitive to the contrast at the boundaries of the black, gray, and white areas, allowing you to see the striped pattern.

Both processes—one that diminishes contrast, and one that enhances contrast—would seem to play a part in your overall perception of the Sliding Gray Step phenomenon.

When all is said and done, however, a big idea to take away is that your perception of an object can be profoundly affected by its surroundings. As demonstrated in this Snack, the perceived color of an object can be significantly affected by the color of its background, or the colors adjacent to it.