Have you ever noticed and wondered why adjacent areas of slightly different solid colors seem to have gradual brightness or color changes? This is an illusion created by our brain, which is a contrast enhancement to help us in discerning slightly different color hues from eachother. The bands of apparent color gradients are called Mach bands and can sometimes be seen in nature, such as a hazy sky over a mountainous region. Mach bands can also appear at the boundaries of sharp shadows on the ground.
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