TheBullseye illusion is ageometricoptical illusion.

In Figures 1 to 4, the green circle appears larger than the blue ring. In reality, however, both areas have the same area.

• 
  Figure 1
• 
  Figure 2
• 
  Figure 3
• 
  Figure 4

The blue ring is formed by the difference in area between a larger circle with radius c and a smaller circle with radius b.

Let the green circle have radius a. Then the condition for the equality of the two areas is:

${\displaystyle \pi a^2=\pi c^2-\pi b^2}$, i.e.,

${\displaystyle a=\sqrt{c^2-b^2}}$.

For the special case${\displaystyle a=b}$ (Figure 4), the condition for the equality of the two areas is:

${\displaystyle \pi b^2=\pi c^2-\pi b^2}$, i.e.,

${\displaystyle a=b=\frac{c}{\sqrt{2}}}$.

In this case, the larger circle is completely decomposed into the smaller green circle and the blue ring of equal area.^[1][2]

• Bullseye illusionWolfram MathWorld
• Bullseye illusion (interactive demonstration)Wolfram Demonstrations Project
• Bullseye optical illusion frommath-fail.com

• Optical illusions