You can make these gray stripes look lighter or darker just by changing their positions on the striped background

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To Do and Notice

-Click on the left or right arrows to shift the gray bars left or right. Notice that when the gray stripes are on the top of the black bars they appear lighter and when they are between the black bars they appear darker.

-Center the gray stripes so they are half-on and half-off the black bars. You can do this either with the arrows or by clicking the gray circular button. Notice that the two sets of gray stripes appear the same.

-Click and hold the striped button to make the black bars disappear. Notice that the two sets of gray stripes appear the same.

What's Going On?

The gray stripes on both sides are the same shade. When you change the position of the stripes, they seem to get lighter and 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, a 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 the retina take a look at the big picture, receiving information from a large area of the retina. These cells blend light from several stripes and react as if the light were mixed together. When there are white stripes on either side of the gray stripes, you see pale gray - a mixture of white and gray. When the gray stripes are surrounded by black stripes, you see dark gray - a mixture of black and gray.

Even though some nerve cells blend the light from the stripes together, other nerve cells receive information from a small area of the retina. These cells note the contrast between the gray stripes, the white stripes, and the black stripes, allowing you to see the striped pattern.