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The Delboeuf Illusions work on the relative size perception of our mind. This particular illusion is one of the simplest that can even be used to teach your children about the concept of relative size. The black dot in the left image appears to be bigger than the black dot on the right However, both the dots are of same size. It is due to the surrounding circle that the size seems to differ even if that's not true.

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When you move your head back and forth, you find the circles rotating in the opposite direction. The reason for motion is the background. Why they are moving in different direction? This is due to the placement of bits that are forming the circles. Look closely, they are placed in opposite directions. This is the reason why they appear to be rotating in different direction.

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Now this effect is commonly used in so many basic illusions as well as complex ones. The logic behind it is that the tinted orange color wave-lines are creating an illusion on the background. The tint effect cannot be seen with the purple colored lines. While purple is promoting the white background, the orange color is creating its own tint in the background.

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