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Gray Step

Science Snack
Gray Step
Without a boundary, it’s hard to distinguish different shades of gray.
Gray Step
Without a boundary, it’s hard to distinguish different shades of gray.

Two slightly different shades of the same color may look different when there’s a sharp boundary between them. But if the boundary is obscured, the two shades may be indistinguishable.

Tools and Materials
  • The full-page Gray Step image supplied below, either printed out or viewed directly on the computer
  • Stiff cardboard or other backing
  • Glue or tape
  • Scissors
  • Yarn or twine to make a fuzzy “horse tail”
  1. Click to enlarge the Gray Step image below. Print the image from your web browser and glue or tape it to your stiff backing or try this activity using the image on your screen.
  2. Make a “horse tail” by knotting together several lengths of yarn or twine. The horse tail needs to be long enough to cover the central boundary in the Gray Step image.
  3. Attach the horse tail to the top of the image and let it hang down to cover the boundary.
To Do and Notice

If you are looking at the enlarged image on your screen, place the horse tail you made over the boundary between the gray halves. (You can also use a pencil or the side of your hand to cover the boundary.) What do you see?

If you are using the print out, position the horse tail so that it covers the boundary line and ask  what they see. Most people will see a uniformly gray piece of paper with a rope hanging down the middle.

Lift the tail and ask again. Most people will see two uniform areas, each a different shade of gray.

In this Snack, a fuzzy cord (the “horse tail”) obscures the boundary between two gray areas. You see one uniform gray area when the horse tail is in place, and two different gray areas when the horse tail is removed. But you never see the truth: Both gray areas are actually identical to one another, grading from lighter gray at one edge to darker gray at the other.

What’s Going On?

Without the horse tail in place, the right and left sides of the rectangle will look different. Actually, they’re the same, but graduated in color. At the right edge, both rectangles are dark gray. Both become lighter toward the left. Where the rectangles meet, the darkest part of one rectangle contrasts sharply with the lightest part of the other, so you see a distinct edge. When the edge is covered, however, the two regions look the same uniform shade of gray (in general, your brain ignores slight gradations in gray levels).

It’s often difficult to distinguish between different shades of gray or shades of the same color when no sharp edge divides them. This is true even though a sensitive light meter would show that the different shades are reflecting different amounts of light to your eyes. It's not that your eyes lack the necessary sensitivity to detect the difference: with an edge between the two shades, the difference is obvious.

Your eye-brain system, however, condenses the information it obtains from more than a hundred million light-detecting rods and cones in the retina in order to send the information to over a million neurons to your brain. Your eye-brain system enhances the ratio of reflected light at edges. If one region of the retina is stimulated by light, lateral connections turn down the sensitivity of adjacent regions. This effect is called lateral inhibition. Conversely, if one region is in the dark, the sensitivity of adjacent regions is increased. This means that a dark region next to a light region looks even darker, and vice versa. As a result, your visual system is most sensitive to changes in brightness and color.

When the horse tail is absent and the normal boundary is visible, lateral inhibition enhances the contrast between the two shades of gray. The bright side appears brighter and the dark side darker. When the tail is in place, the boundary between the two different grays is spread apart across the retina so that it no longer falls on adjacent regions. Lateral inhibition then does not help distinguish between the different shades, and the eye-brain system judges them to be the same.

Going Further

If a viewer claims they can tell which side is darker when the horse tail is in place, try rotating the Gray Step image and flipping the horse tail when they aren't looking and see if they can tell the difference.

If you want to prove that the two sides of the Gray Step image are identically shaded from one side to the other, you can cut the printed image along the boundary line and compare the two halves directly by overlapping them and orienting them in different ways. 

You can also make a simple mask from a piece of white paper and a single-hole punch. Punch two holes in the paper so they’re spaced about the same distance apart as the width of one of the two halves. When you put this sheet of paper over the printed image, a small gray circle from each side will show through the holes. Push the white paper back and forth across the Gray Step master and notice that, no matter where you move it, the two areas viewed through the punches are always matching shades of gray.

Another easy Gray Step demonstration can be constructed out of paint samples from a hardware store. Some paint brands provide good-sized individual color chips. Experiment with how two colors can be different and yet appear the same when the boundary between them is obscured.