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Minor Hall has a large patio on the north side. Go to there and look toward the Campanile (Sather Tower).

Hold your hand up (as the nerd who took the photograph has done) such that you can’t see the parts above the clocks. Do the illuminated and shaded sides of the tower look equally bright? That is, do the reflectances of the bricks on the two sides appear equal? Most people see the reflectances as similar.

Now see if you can through conscious effort get the vertex of the tower to flip perceptually. What we mean is this. The vertex is obviously convex, but you may be able to imagine it as concave. If you can get it to flip like that, what happens to the apparent brightnesses of the left and right sides of the tower? Doesn’t the right side (if it is the shaded side) now look darker?

The explanation for this illusion concerns the way in which the brain takes the known position of the light source (in this case the sun) into account when estimating the reflectance of a surface. This illusion was first described by Ernst Mach, the Austrian physicist and philosopher.

For more information on Sather Tower and its history, refer to:
Brief History
UC History Digital Archives

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