20. The Barberpole Illusion A barber's pole is a type of sign used by barbers to signify the place or shop where they perform their craft. The trade sign is, by a tradition dating back to the Middle Ages, a staff or pole with a helix of colored stripes, usually red, white and blue in the United States. The pole may be stationary or may revolve, often with the aid of an electric motor. You can begin to see why in the demo on the left: focus on the vertical slot and the grating seems to be moving vertically (as in the barber’s pole).  Focus on the horizontal slot and in a moment the grating may seem to move horizontally.  Behind the round hole, it tends to look as if moving diagonally. In the vertical slot that is taller then wider, the motion appears vertical. In the horizontal slot that is wider than taller, the motion appears horizontal. The motion appears in the direction of the greater length. In a circle, there is no bias and the motion appears diagonally. The barber’s sign shows a famous illusion. The cylinder is rotating horizontally around a vertical axis, but the stripes look as if they are rising – which would be impossible, unless you had some long pole sliding through the cylinder. Click on picture and start the animation Click on picture and start the animation www.mastermathmentor.com

21. Broken Arch Illusion This a pointed Gothic arch in Germany. It’s shape, however, is broken by the vertical brown bar. The two stone arcs going up behind it clearly do not meet at the top. Or do they? With the brown bar somewhat transparent, you can see that they certainly do meet. This is an example of the Poggendorff Illusion. www.mastermathmentor.com

22. Blur Illusion You probably see a random array of blocks having little meaning. But blurring the image reveals an interesting message which is decidedly not true! www.mastermathmentor.com

23. Motion Binding Illusion The motion of the blue lines remains the same whether the green squares are present or not. But our perception changes dramatically. Now the blue lines suddenly outline a diamond, moving in a circular fashion! How would you describe the motion of the 4 lines? Most people see the blue lines moving in opposite pairs. The mechanism at work here is known as “motion binding”. When the edges of the diamond are covered by squares with the same color as the background (here, grey) there is no information on the vertices of the square. Now the ends of the line become a property of the line with insufficient information to detect the circular movement. www.mastermathmentor.com

24. Kanisza Illusion z In the figure to the right, the blue Pac-Men figures define an inner triangle that seems brighter than its surrounding area and seems to hover above the background. We move the blue Pac-Men in just slightly and the illusion disappears. www.mastermathmentor.com

Sources 20. Barberpole - https://en.wikipedia.org/wiki/Barberpole_illusion, https://www.opticalillusion.net/optical-illusions/the-barber-pole-illusion/ 21. Broken Arch - https://michaelbach.de/ot/ang-arch/index.html 22. Blur https://michaelbach.de/ot/cog-blur_nsx/index.html 23. Motion Binding - https://www.youtube.com/watch?v=ycYsuRQd7No 24. Kanisza - https://michaelbach.de/ot/cog-kanizsa/index.html www.mastermathmentor.com